commit: use generations in paint_down_to_common()

Define compare_commits_by_gen_then_commit_date(), which uses generation
numbers as a primary comparison and commit date to break ties (or as a
comparison when both commits do not have computed generation numbers).

Since the commit-graph file is closed under reachability, we know that
all commits in the file have generation at most GENERATION_NUMBER_MAX
which is less than GENERATION_NUMBER_INFINITY.

This change does not affect the number of commits that are walked during
the execution of paint_down_to_common(), only the order that those
commits are inspected. In the case that commit dates violate topological
order (i.e. a parent is "newer" than a child), the previous code could
walk a commit twice: if a commit is reached with the PARENT1 bit, but
later is re-visited with the PARENT2 bit, then that PARENT2 bit must be
propagated to its parents. Using generation numbers avoids this extra
effort, even if it is somewhat rare.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -640,6 +640,24 @@ static int compare_commits_by_author_date(const void *a_, const void *b_,
 	return 0;
 }
+int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused)
+{
+	const struct commit *a = a_, *b = b_;
+
+	/* newer commits first */
+	if (a->generation < b->generation)
+		return 1;
+	else if (a->generation > b->generation)
+		return -1;
+
+	/* use date as a heuristic when generations are equal */
+	if (a->date < b->date)
+		return 1;
+	else if (a->date > b->date)
+		return -1;
+	return 0;
+}
+
 int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused)
 {
 	const struct commit *a = a_, *b = b_;
@@ -789,7 +807,7 @@ static int queue_has_nonstale(struct prio_queue *queue)
 /* all input commits in one and twos[] must have been parsed! */
 static struct commit_list *paint_down_to_common(struct commit *one, int n, struct commit **twos)
 {
-	struct prio_queue queue = { compare_commits_by_commit_date };
+	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
 	struct commit_list *result = NULL;
 	int i;

commit.h

@@ -341,6 +341,7 @@ extern int remove_signature(struct strbuf *buf);
 extern int check_commit_signature(const struct commit *commit, struct signature_check *sigc);
 int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused);
+int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused);
 LAST_ARG_MUST_BE_NULL
 extern int run_commit_hook(int editor_is_used, const char *index_file, const char *name, ...);