merge: use repo_in_merge_bases for octopus up-to-date check

The octopus merge path checks whether each remote head is already
an ancestor of HEAD by computing all merge-bases via
repo_get_merge_bases() and comparing the first result's OID to
the remote head.  This is more expensive than necessary:
repo_get_merge_bases() calls paint_down_to_common() with
min_generation=0, performs the full STALE drain, and may run
remove_redundant(), when all we need is a yes/no reachability
answer.

Replace this with repo_in_merge_bases(), which answers the
is-ancestor question directly.  When generation numbers are
available, repo_in_merge_bases() uses can_all_from_reach() -- a
DFS bounded by generation number that stops as soon as the target
is found or ruled out, without entering paint_down_to_common() at
all.  Without generation numbers, it still benefits from a tighter
min_generation floor.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1735,21 +1735,11 @@ int cmd_merge(int argc,
 		struct commit_list *j;
 
 		for (j = remoteheads; j; j = j->next) {
-			struct commit_list *common_one = NULL;
-			struct commit *common_item;
-
-			/*
-			 * Here we *have* to calculate the individual
-			 * merge_bases again, otherwise "git merge HEAD^
-			 * HEAD^^" would be missed.
-			 */
-			if (repo_get_merge_bases(the_repository, head_commit,
-						 j->item, &common_one) < 0)
+			int ret = repo_in_merge_bases(the_repository,
+						      j->item, head_commit);
+			if (ret < 0)
 				exit(128);
-
-			common_item = common_one->item;
-			commit_list_free(common_one);
-			if (!oideq(&common_item->object.oid, &j->item->object.oid)) {
+			if (!ret) {
 				up_to_date = 0;
 				break;
 			}

t/t6408-merge-up-to-date.sh

@@ -89,4 +89,14 @@ test_expect_success 'merge fast-forward octopus' '
 	test "$expect" = "$current"
 '
 
+test_expect_success 'merge octopus already up to date' '
+
+	git reset --hard c2 &&
+	test_tick &&
+	git merge c0 c1 &&
+	expect=$(git rev-parse c2) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+'
+
 test_done