commit-reach: guard !FIND_ALL early exit with generation ordering check

When paint_down_to_common() falls back to commit-date ordering (for
v1 commit graphs without corrected commit dates), the !FIND_ALL early
exit incorrectly fires.  The exit assumes the queue is generation-
ordered, so the first RESULT commit found must be the shallowest.
With date ordering this is not guaranteed: a closer merge base with
a lower committer date (clock skew) may still be in the queue behind
deeper commits.

Add a gen_ordered flag that is cleared when the date fallback fires,
and require it for the early exit.

Update the test from the previous commit to test_expect_success.

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

Author: spkrka

commit-reach.c

@@ -59,11 +59,14 @@ static int paint_down_to_common(struct repository *r,
 {
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
 	int i;
+	int gen_ordered = 1;
 	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
-	if (!min_generation && !corrected_commit_dates_enabled(r))
+	if (!min_generation && !corrected_commit_dates_enabled(r)) {
 		queue.compare = compare_commits_by_commit_date;
+		gen_ordered = 0;
+	}
 
 	one->object.flags |= PARENT1;
 	if (!n) {
@@ -98,11 +101,12 @@ static int paint_down_to_common(struct repository *r,
 				commit->object.flags |= RESULT;
 				tail = commit_list_append(commit, tail);
 				/*
-				 * The queue is generation-ordered; no
-				 * remaining common ancestor can be a
+				 * When the queue is generation-ordered,
+				 * no remaining common ancestor can be a
 				 * descendant of this one.
 				 */
 				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
+				    gen_ordered &&
 				    generation < GENERATION_NUMBER_INFINITY)
 					break;
 			}

t/t6600-test-reach.sh

@@ -958,7 +958,7 @@ test_expect_success 'merge-base without --all is one of --all results' '
 	grep -F -f single all
 '
 
-test_expect_failure 'merge-base without --all, clock skew, v1 commit-graph' '
+test_expect_success 'merge-base without --all, clock skew, v1 commit-graph' '
 	git rev-parse skew-M2 >expect &&
 	merge_base_all_modes skew-P1 skew-P2
 '