merge-recursive: allow storing conflict hunks in index

Add a --conflicts-in-index option to merge-recursive, which instructs
it to always store the 3-way merged result in the index.  (Normally it
only does so in recursive invocations, but not for the final result.)

This serves as a building block for the "remerge diff" feature coming
up in a subsequent patch.  The external option lets us easily use it
from tests, where we'd otherwise need a new test-* helper to access
the feature.

Furthermore, it might occasionally be useful for scripts that want to
look at the result of invoking git-merge without tampering with the
worktree.  They could already get the _conflicts_ with --index-only,
but not (conveniently) the conflict-hunk formatted files that would
normally be written to the worktree.

Signed-off-by: Thomas Rast <tr@thomasrast.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/merge-strategies.txt b/Documentation/merge-strategies.txt
index 7716fda9c1..19f3a5d25f 100644
--- a/Documentation/merge-strategies.txt
+++ b/Documentation/merge-strategies.txt
@@ -96,6 +96,11 @@ index-only;;
 	Write the merge result only to the index; do not touch the
 	worktree.
 
+conflicts-in-index;;
+	For conflicted files, write 3-way merged contents with
+	conflict hunks to the index, instead of leaving their entries
+	unresolved.
+
 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 d54bac2bd8..20d3051a78 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -743,6 +743,8 @@ static void update_file_flags(struct merge_options *o,
 			      int update_cache,
 			      int update_wd)
 {
+	if (o->conflicts_in_index)
+		update_cache = 1;
 	if (o->call_depth || o->no_worktree)
 		update_wd = 0;
 
@@ -2110,6 +2112,8 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 	}
 	else if (!strcmp(s, "index-only"))
 		o->no_worktree = 1;
+	else if (!strcmp(s, "conflicts-in-index"))
+		o->conflicts_in_index = 1;
 	else
 		return -1;
 	return 0;
diff --git a/merge-recursive.h b/merge-recursive.h
index d8dd7a1211..9b8e20bee2 100644
--- a/merge-recursive.h
+++ b/merge-recursive.h
@@ -16,6 +16,7 @@ struct merge_options {
 	unsigned buffer_output : 1;
 	unsigned renormalize : 1;
 	unsigned no_worktree : 1; /* do not touch worktree */
+	unsigned conflicts_in_index : 1; /* index will contain conflict hunks */
 	long xdl_opts;
 	int verbosity;
 	int diff_rename_limit;
diff --git a/t/t3030-merge-recursive.sh b/t/t3030-merge-recursive.sh
index be07705b6a..39841a933a 100755
--- a/t/t3030-merge-recursive.sh
+++ b/t/t3030-merge-recursive.sh
@@ -310,6 +310,26 @@ test_expect_success 'merge-recursive --index-only' '
 	test_cmp expected-diff actual-diff
 '
 
+test_expect_success 'merge-recursive --index-only --conflicts-in-index' '
+	# first pass: do a merge as usual to obtain "expected"
+	rm -fr [abcd] &&
+	git checkout -f "$c2" &&
+	test_expect_code 1 git merge-recursive "$c0" -- "$c2" "$c1" &&
+	git add [abcd] &&
+	git ls-files -s >expected &&
+	# second pass: actual test
+	rm -fr [abcd] &&
+	git checkout -f "$c2" &&
+	test_expect_code 1 \
+		git merge-recursive --index-only --conflicts-in-index \
+		"$c0" -- "$c2" "$c1" &&
+	git ls-files -s >actual &&
+	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] &&