diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index d7f9121f2f..791fd8260c 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh
@@ -788,42 +788,6 @@ ensure_valid_ref_format () {
 		die "fatal: '$1' does not look like a ref"
 }
 
-# Usage: should_ignore_subtree_split_commit REV
-#
-# Check if REV is a commit from another subtree and should be
-# ignored from processing for splits
-should_ignore_subtree_split_commit () {
-	assert test $# = 1
-
-	git show \
-		--no-patch \
-		--no-show-signature \
-		--format='%(trailers:key=git-subtree-dir,key=git-subtree-mainline)' \
-		"$1" |
-	(
-	have_mainline=
-	subtree_dir=
-
-	while read -r trailer val
-	do
-		case "$trailer" in
-		git-subtree-dir:)
-			subtree_dir="${val%/}" ;;
-		git-subtree-mainline:)
-			have_mainline=y ;;
-		esac
-	done
-
-	if test -n "${subtree_dir}" &&
-		test -z "${have_mainline}" &&
-		test "${subtree_dir}" != "$arg_prefix"
-	then
-		return 0
-	fi
-	return 1
-	)
-}
-
 # Usage: process_split_commit REV PARENTS
 process_split_commit () {
 	assert test $# = 2
@@ -1015,19 +979,7 @@ cmd_split () {
 	eval "$grl" |
 	while read rev parents
 	do
-		if should_ignore_subtree_split_commit "$rev"
-		then
-			continue
-		fi
-		parsedparents=''
-		for parent in $parents
-		do
-			if ! should_ignore_subtree_split_commit "$parent"
-			then
-				parsedparents="$parsedparents$parent "
-			fi
-		done
-		process_split_commit "$rev" "$parsedparents"
+		process_split_commit "$rev" "$parents"
 	done || exit $?
 
 	latest_new=$(cache_get latest_new) || exit $?
diff --git a/contrib/subtree/t/t7900-subtree.sh b/contrib/subtree/t/t7900-subtree.sh
index 8024703cad..18d2b56448 100755
--- a/contrib/subtree/t/t7900-subtree.sh
+++ b/contrib/subtree/t/t7900-subtree.sh
@@ -433,8 +433,9 @@ test_expect_success 'split sub dir/ with --rejoin' '
 		git fetch ./"sub proj" HEAD &&
 		git subtree merge --prefix="sub dir" FETCH_HEAD &&
 		split_hash=$(git subtree split --prefix="sub dir" --annotate="*") &&
-		git subtree split --prefix="sub dir" --annotate="*" --rejoin &&
-		test "$(last_commit_subject)" = "Split '\''sub dir/'\'' into commit '\''$split_hash'\''"
+		git subtree split --prefix="sub dir" --annotate="*" -b spl --rejoin &&
+		test "$(last_commit_subject)" = "Split '\''sub dir/'\'' into commit '\''$split_hash'\''" &&
+		test "$(git rev-list --count spl)" -eq 5
 	)
 '
 
@@ -449,8 +450,7 @@ test_expect_success 'split sub dir/ with --rejoin' '
 # 	- Perform 'split' on subtree B
 # 	- Create new commits with changes to subtree A and B
 # 	- Perform split on subtree A
-# 	- Check that the commits in subtree B are not processed
-#			as part of the subtree A split
+# 	- Check for expected history
 test_expect_success 'split with multiple subtrees' '
 	subtree_test_create_repo "$test_count" &&
 	subtree_test_create_repo "$test_count/subA" &&
@@ -464,18 +464,25 @@ test_expect_success 'split with multiple subtrees' '
 	git -C "$test_count" subtree add --prefix=subADir FETCH_HEAD &&
 	git -C "$test_count" fetch ./subB HEAD &&
 	git -C "$test_count" subtree add --prefix=subBDir FETCH_HEAD &&
+	test "$(git -C "$test_count" rev-list --count main)" -eq 7 &&
 	test_create_commit "$test_count" subADir/main-subA1 &&
 	test_create_commit "$test_count" subBDir/main-subB1 &&
 	git -C "$test_count" subtree split --prefix=subADir \
-		--squash --rejoin -m "Sub A Split 1" &&
+		--squash --rejoin -m "Sub A Split 1" -b a1 &&
+	test "$(git -C "$test_count" rev-list --count main..a1)" -eq 1 &&
 	git -C "$test_count" subtree split --prefix=subBDir \
-		--squash --rejoin -m "Sub B Split 1" &&
+		--squash --rejoin -m "Sub B Split 1" -b b1 &&
+	test "$(git -C "$test_count" rev-list --count main..b1)" -eq 1 &&
 	test_create_commit "$test_count" subADir/main-subA2 &&
 	test_create_commit "$test_count" subBDir/main-subB2 &&
 	git -C "$test_count" subtree split --prefix=subADir \
-		--squash --rejoin -m "Sub A Split 2" &&
-	test "$(git -C "$test_count" subtree split --prefix=subBDir \
-		--squash --rejoin -d -m "Sub B Split 1" 2>&1 | grep -w "\[1\]")" = ""
+		--squash --rejoin -m "Sub A Split 2" -b a2 &&
+	test "$(git -C "$test_count" rev-list --count main..a2)" -eq 2 &&
+	test "$(git -C "$test_count" rev-list --count a1..a2)" -eq 1 &&
+	git -C "$test_count" subtree split --prefix=subBDir \
+		--squash --rejoin -d -m "Sub B Split 1" -b b2 &&
+	test "$(git -C "$test_count" rev-list --count main..b2)" -eq 2 &&
+	test "$(git -C "$test_count" rev-list --count b1..b2)" -eq 1
 '
 
 # When subtree split-ing a directory that has other subtree
@@ -499,6 +506,7 @@ do
 			test_path_is_file subA/file1.t &&
 			test_path_is_file subA/subB/file2.t &&
 			git subtree split --prefix=subA --branch=bsplit &&
+			test "$(git rev-list --count bsplit)" -eq 2 &&
 			git checkout bsplit &&
 			test_path_is_file file1.t &&
 			test_path_is_file subB/file2.t &&
@@ -511,6 +519,7 @@ do
 				--prefix=subA/subB mksubtree &&
 			test_path_is_file subA/subB/file3.t &&
 			git subtree split --prefix=subA --branch=bsplit &&
+			test "$(git rev-list --count bsplit)" -eq 3 &&
 			git checkout bsplit &&
 			test_path_is_file file1.t &&
 			test_path_is_file subB/file2.t &&
@@ -519,6 +528,67 @@ do
 	'
 done
 
+# Usually,
+#
+#    git subtree merge -P subA --squash f00...
+#
+# makes two commits, in this order:
+#
+# 1. Squashed 'subA/' content from commit f00...
+# 2. Merge commit (1) as 'subA'
+#
+# Commit 1 updates the subtree but does *not* rewrite paths.
+# Commit 2 rewrites all trees to start with `subA/`
+#
+# Commit 1 either has no parents or depends only on other
+# "Squashed 'subA/' content" commits.
+#
+# For merge without --squash, subtree produces just one commit:
+# a merge commit with git-subtree trailers.
+#
+# In either case, if the user rebases these commits, they will
+# still have the git-subtree-* trailers… but will NOT have
+# the layout described above.
+#
+# Test that subsequent `git subtree split` are not confused by this.
+test_expect_success 'split with rebased subtree commit' '
+	subtree_test_create_repo "$test_count" &&
+	(
+		cd "$test_count" &&
+		test_commit file0 &&
+		test_create_subtree_add \
+			. mksubtree subA file1 --squash &&
+		test_path_is_file subA/file1.t &&
+		mkdir subB &&
+		test_commit subB/bfile &&
+		git commit --amend -F - <<'EOF' &&
+Squashed '\''subB/'\'' content from commit '\''badf00da911bbe895347b4b236f5461d55dc9877'\''
+
+Simulate a cherry-picked or rebased subtree commit.
+
+git-subtree-dir: subB
+git-subtree-split: badf00da911bbe895347b4b236f5461d55dc9877
+EOF
+		test_commit subA/file2 &&
+		test_commit subB/bfile2 &&
+		git commit --amend -F - <<'EOF' &&
+Split '\''subB/'\'' into commit '\''badf00da911bbe895347b4b236f5461d55dc9877'\''
+
+Simulate a cherry-picked or rebased subtree commit.
+
+git-subtree-dir: subB
+git-subtree-mainline: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+git-subtree-split: badf00da911bbe895347b4b236f5461d55dc9877
+EOF
+		git subtree split --prefix=subA --branch=bsplit &&
+		git checkout bsplit &&
+		test_path_is_file file1.t &&
+		test_path_is_file file2.t &&
+		test "$(last_commit_subject)" = "subA/file2" &&
+		test "$(git rev-list --count bsplit)" -eq 2
+	)
+'
+
 test_expect_success 'split sub dir/ with --rejoin from scratch' '
 	subtree_test_create_repo "$test_count" &&
 	test_create_commit "$test_count" main1 &&
@@ -1597,6 +1667,116 @@ test_expect_success 'push split to subproj' '
 	)
 '
 
+# --ignore-joins must ignore mainline content outside of the
+# subtree. This test verifies that the logic in
+# `find_existing_splits()` correctly handles a `git subtree add`
+# In this test, the split history must not contain a commit titled
+#
+#     Add 'sub/' from commit ...
+#
+# see: dd21d43b58 (subtree: make --ignore-joins pay
+#      attention to adds, 2018-09-28)
+test_expect_success 'split --ignore-joins respects subtree add' '
+	subtree_test_create_repo "$test_count" &&
+	(
+		cd "$test_count" &&
+		test_commit main_must_not_be_in_subtree &&
+		test_create_subtree_add . mksubtree sub sub1 &&
+		test_commit sub/sub2 &&
+		test_commit main_must_not_be_in_subtree2 &&
+		git subtree split --prefix sub -b first_split --rejoin &&
+		test_commit sub/sub3 &&
+		no_ignore_joins="$(git subtree split --prefix sub -b no_ignore_joins)" &&
+		ignore_joins="$(git subtree split --prefix sub --ignore-joins -b ignore_joins)" &&
+		git checkout ignore_joins &&
+		test_path_is_file sub1.t &&
+		test_path_is_file sub2.t &&
+		test_path_is_file sub3.t &&
+		! test_path_is_file main_must_not_be_in_subtree.t &&
+		! test_path_is_file main_must_not_be_in_subtree2.t &&
+		test -z "$(git log -1 --grep "Add '''sub/''' from commit" ignore_joins)" &&
+		test "$no_ignore_joins" = "$ignore_joins" &&
+		test "$(git rev-list --count ignore_joins)" -eq 3;
+	)
+'
+
+# split excludes commits reachable from any previous --rejoin.
+# These ignored commits can still be the basis for new work
+# after the --rejoin. These commits must be processed, even
+# if they are excluded. Otherwise, the split history will be
+# incorrect.
+#
+# here, the merge
+#
+#     git merge --no-ff new_work_based_on_prejoin
+#
+# doesn't contain any subtree changes and so should not end
+# up in the split history. this subtree should be flat,
+# with no merges.
+#
+# see: 315a84f9aa (subtree: use commits before rejoins for
+#      splits, 2018-09-28)
+test_expect_success 'split links out-of-tree pre --rejoin commits with post --rejoin commits' '
+	subtree_test_create_repo "$test_count" &&
+	(
+		cd "$test_count" &&
+		test_commit main_must_not_be_in_subtree &&
+		mkdir sub &&
+		test_commit sub/sub1 &&
+		test_commit sub/sub2 &&
+		git subtree split --prefix sub --rejoin &&
+		test "$(git rev-list --count HEAD)" -eq 6 &&
+		git checkout sub/sub1 &&
+		git checkout -b new_work_based_on_prejoin &&
+		test_commit main_must_not_be_in_subtree2 &&
+		git checkout main &&
+		git merge --no-ff new_work_based_on_prejoin &&
+		test_commit sub/sub3 &&
+		git subtree split -d --prefix sub -b second_split &&
+		git checkout second_split &&
+		test_path_is_file sub1.t &&
+		test_path_is_file sub2.t &&
+		test_path_is_file sub3.t &&
+		! test_path_is_file main_must_not_be_in_subtree.t &&
+		! test_path_is_file main_must_not_be_in_subtree2.t &&
+		test "$(git rev-list --count --merges second_split)" -eq 0 &&
+		test "$(git rev-list --count second_split)" -eq 3;
+	)
+'
+
+# split must keep merge commits with unrelated histories, even
+# if both parents are treesame. When deciding whether or not
+# to eliminate a parent, copy_or_skip compares the merge-base
+# of each parent.
+#
+# in the split_of_merges branch:
+#
+#   * expect 4 commits
+#   * HEAD~ must be a merge
+#
+# see: 68f8ff8151 (subtree: improve decision on merges kept
+#      in split, 2018-09-28)
+test_expect_success 'split preserves merges with unrelated history' '
+	subtree_test_create_repo "$test_count" &&
+	(
+		cd "$test_count" &&
+		test_commit main_must_not_be_in_subtree &&
+		mkdir sub &&
+		test_commit sub/sub1 &&
+		git checkout --orphan new_history &&
+		git checkout sub/sub1 -- . &&
+		git add . &&
+		git commit -m "treesame history but not a merge-base" &&
+		git checkout main &&
+		git merge --allow-unrelated-histories --no-ff new_history &&
+		test "$(git rev-parse "HEAD^1^{tree}")" = "$(git rev-parse "HEAD^2^{tree}")" &&
+		test_commit sub/sub2 &&
+		git subtree split -d --prefix sub -b split_of_merges &&
+		test "$(git rev-list --count split_of_merges)" -eq 4 &&
+		test -n "$(git rev-list --merges HEAD~)";
+	)
+'
+
 #
 # This test covers 2 cases in subtree split copy_or_skip code
 # 1) Merges where one parent is a superset of the changes of the other