merge-file: honor merge.conflictStyle outside of a repository

When running outside a repository, git merge-file ignores the
merge.conflictStyle configuration variable entirely. Since the
function receives `repo` from the caller (which is NULL outside a
repository), and repo_config() falls back to reading system and user
configuration when passed NULL, pass `repo` to repo_config()
unconditionally.

Also document that merge.conflictStyle is honored.

Signed-off-by: Yannik Tausch <dev@ytausch.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-file.adoc

@@ -85,6 +85,9 @@ object store and the object ID of its blob is written to standard output.
 
 --zdiff3::
 	Show conflicts in "zdiff3" style.
++
+The `--diff3` and `--zdiff3` options default to the value of the
+`merge.conflictStyle` configuration variable (see linkgit:git-config[1]).
 
 --ours::
 --theirs::

builtin/merge-file.c

@@ -60,7 +60,7 @@ static int diff_algorithm_cb(const struct option *opt,
 int cmd_merge_file(int argc,
 		   const char **argv,
 		   const char *prefix,
-		   struct repository *repo UNUSED)
+		   struct repository *repo)
 {
 	const char *names[3] = { 0 };
 	mmfile_t mmfs[3] = { 0 };
@@ -95,12 +95,10 @@ int cmd_merge_file(int argc,
 	xmp.style = 0;
 	xmp.favor = 0;
 
-	if (startup_info->have_repository) {
-		/* Read the configuration file */
-		repo_config(the_repository, git_xmerge_config, NULL);
-		if (0 <= git_xmerge_style)
-			xmp.style = git_xmerge_style;
-	}
+	/* Read the configuration file */
+	repo_config(repo, git_xmerge_config, NULL);
+	if (0 <= git_xmerge_style)
+		xmp.style = git_xmerge_style;
 
 	argc = parse_options(argc, argv, prefix, options, merge_file_usage, 0);
 	if (argc != 3)

t/t6403-merge-file.sh

@@ -428,6 +428,42 @@ test_expect_success '"diff3 -m" style output (2)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'merge.conflictStyle honored outside repo' '
+	test_config_global merge.conflictStyle diff3 &&
+	cat >nongit-base <<-\EOF &&
+	line1
+	original
+	line3
+	EOF
+	cat >nongit-ours <<-\EOF &&
+	line1
+	ours
+	line3
+	EOF
+	cat >nongit-theirs <<-\EOF &&
+	line1
+	theirs
+	line3
+	EOF
+	cat >expect <<-\EOF &&
+	line1
+	<<<<<<< ours
+	ours
+	||||||| base
+	original
+	=======
+	theirs
+	>>>>>>> theirs
+	line3
+	EOF
+	test_must_fail nongit git merge-file -p \
+		-L ours -L base -L theirs \
+		"$PWD/nongit-ours" \
+		"$PWD/nongit-base" \
+		"$PWD/nongit-theirs" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'marker size' '
 	cat >expect <<-\EOF &&
 	Dominus regit me,