From a2a92ab17fe120f6b28d123ac2fd41f066f0de1e Mon Sep 17 00:00:00 2001
From: Junio C Hamano <junkio@cox.net>
Date: Wed, 29 Nov 2006 18:53:13 -0800
Subject: [PATCH] git-merge: preserve and merge local changes when doing fast
 forward

The idea and the logic are identical to what "checkout -m" does
when switching the branches.  Instead of refusing the two-way
merge, perform the three-way merge between the old head, the
working tree and the new head, and leave the (potentially
conflicted) merge result in the working tree.  When the resulting
conflict were too much to handle for the user, there is no easy
way to get that back, so they are stashed away in $GIT_DIR/LOCAL_DIFF
file.  We do the same for "git checkout -m".

If this turns out to be a sane thing to do, we probably should
make the common logic between "checkout -m" and this into a
built-in command.

Signed-off-by: Junio C Hamano <junkio@cox.net>
---
 git-checkout.sh |  7 +++++++
 git-merge.sh    | 47 ++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/git-checkout.sh b/git-checkout.sh
index 737abd0c09..5bf399a276 100755
--- a/git-checkout.sh
+++ b/git-checkout.sh
@@ -168,6 +168,9 @@ else
 		exit 1 ;;
 	esac
 
+	# First stash away the local changes
+	git diff-index --binary -p HEAD >"$GIT_DIR"/LOCAL_DIFF
+
 	# Match the index to the working tree, and do a three-way.
     	git diff-files --name-only | git update-index --remove --stdin &&
 	work=`git write-tree` &&
@@ -195,6 +198,10 @@ else
 			sed -e 's/^[0-7]* [0-9a-f]* /'"0 $z40 /"
 			echo "$unmerged"
 		) | git update-index --index-info
+
+		echo >&2 "Conflicts in locally modified files:"
+		git diff --name-only --diff-filter=U >&2
+		echo >&2 "Your local changes are found in $GIT_DIR/LOCAL_DIFF"
 		;;
 	esac
 	exit 0
diff --git a/git-merge.sh b/git-merge.sh
index 272f004622..798aad3576 100755
--- a/git-merge.sh
+++ b/git-merge.sh
@@ -91,6 +91,51 @@ finish () {
 	esac
 }
 
+merge_local_changes () {
+	merge_error=$(git-read-tree -m -u $1 $2 2>&1) || (
+
+		# First stash away the local changes
+		git diff-index --binary -p HEAD >"$GIT_DIR"/LOCAL_DIFF
+
+		# Match the index to the working tree, and do a three-way.
+		git diff-files --name-only |
+		git update-index --remove --stdin &&
+		work=`git write-tree` &&
+		git read-tree --reset -u $2 &&
+		git read-tree -m -u --aggressive $1 $2 $work || exit
+
+		echo >&2 "Carrying local changes forward."
+		if result=`git write-tree 2>/dev/null`
+		then
+			echo >&2 "Trivially automerged."
+		else
+			git merge-index -o git-merge-one-file -a
+		fi
+
+		# Do not register the cleanly merged paths in the index
+		# yet; this is not a real merge before committing, but
+		# just carrying the working tree changes along.
+		unmerged=`git ls-files -u`
+		git read-tree --reset $2
+		case "$unmerged" in
+		'')	;;
+		*)
+			(
+				z40=0000000000000000000000000000000000000000
+				echo "$unmerged" |
+				sed -e 's/^[0-7]* [0-9a-f]* /'"0 $z40 /"
+				echo "$unmerged"
+			) | git update-index --index-info
+
+			echo >&2 "Conflicts in locally modified files:"
+			git diff --name-only --diff-filter=U >&2
+			echo >&2 "Your local changes are found in $GIT_DIR/LOCAL_DIFF"
+			;;
+		esac
+		exit 0
+	)
+}
+
 case "$#" in 0) usage ;; esac
 
 rloga= have_message=
@@ -264,7 +309,7 @@ f,*)
 	echo "Updating $(git-rev-parse --short $head)..$(git-rev-parse --short $1)"
 	git-update-index --refresh 2>/dev/null
 	new_head=$(git-rev-parse --verify "$1^0") &&
-	git-read-tree -u -v -m $head "$new_head" &&
+	merge_local_changes $head $new_head &&
 	finish "$new_head" "Fast forward"
 	dropsave
 	exit 0