merge-recursive: -Xindex-only to leave worktree unchangedtr/merge-recursive-index-only

Using the new no_worktree flag from the previous commit, we can teach
merge-recursive to leave the worktree untouched.  Expose this with a
new strategy option so that scripts can use it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/merge-strategies.txt b/Documentation/merge-strategies.txt
index fb6e593e7c..2934e993f8 100644
--- a/Documentation/merge-strategies.txt
+++ b/Documentation/merge-strategies.txt
@@ -92,6 +92,10 @@ subtree[=<path>];;
 	is prefixed (or stripped from the beginning) to make the shape of
 	two trees to match.
 
+index-only;;
+	Write the merge result only to the index; do not touch the
+	worktree.
+
 octopus::
 	This resolves cases with more than two heads, but refuses to do
 	a complex merge that needs manual resolution.  It is
diff --git a/merge-recursive.c b/merge-recursive.c
index 35be1441db..f59c1d399f 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2096,6 +2096,8 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 		if ((o->rename_score = parse_rename_score(&score)) == -1 || *score != 0)
 			return -1;
 	}
+	else if (!strcmp(s, "index-only"))
+		o->no_worktree = 1;
 	else
 		return -1;
 	return 0;
diff --git a/t/t3030-merge-recursive.sh b/t/t3030-merge-recursive.sh
index 2f96100a5f..2f3a16ca53 100755
--- a/t/t3030-merge-recursive.sh
+++ b/t/t3030-merge-recursive.sh
@@ -296,6 +296,19 @@ test_expect_success 'merge-recursive result' '
 
 '
 
+test_expect_success 'merge-recursive --index-only' '
+
+	rm -fr [abcd] &&
+	git checkout -f "$c2" &&
+	test_expect_code 1 git merge-recursive --index-only "$c0" -- "$c2" "$c1" &&
+	git ls-files -s >actual &&
+	# reuses "expected" from previous test!
+	test_cmp expected actual &&
+	git diff HEAD >actual-diff &&
+	: >expected-diff &&
+	test_cmp expected-diff actual-diff
+'
+
 test_expect_success 'fail if the index has unresolved entries' '
 
 	rm -fr [abcd] &&