merge_bases_many(): split out the logic to paint history

Introduce a new helper function paint_down_to_common() that takes
the same parameters as merge_bases_many(), but without the first
optimization of not painting anything when "one" is one of the
"twos" (or vice versa), and the last clean-up of removing the common
ancestor that is known to be an ancestor of another common one.

This way, the caller of the new function could tell if "one" is
reachable from any of the "twos" by simply looking at the flag bits
of "one".  If (and only if) it is painted in PARENT2, it is
reachable from one of the "twos".

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

commit.c

@@ -581,28 +581,12 @@ static struct commit *interesting(struct commit_list *list)
 	return NULL;
 }
 
-static struct commit_list *merge_bases_many(struct commit *one, int n, struct commit **twos)
+static struct commit_list *paint_down_to_common(struct commit *one, int n, struct commit **twos)
 {
 	struct commit_list *list = NULL;
 	struct commit_list *result = NULL;
 	int i;
 
-	for (i = 0; i < n; i++) {
-		if (one == twos[i])
-			/*
-			 * We do not mark this even with RESULT so we do not
-			 * have to clean it up.
-			 */
-			return commit_list_insert(one, &result);
-	}
-
-	if (parse_commit(one))
-		return NULL;
-	for (i = 0; i < n; i++) {
-		if (parse_commit(twos[i]))
-			return NULL;
-	}
-
 	one->object.flags |= PARENT1;
 	commit_list_insert_by_date(one, &list);
 	for (i = 0; i < n; i++) {
@@ -643,9 +627,34 @@ static struct commit_list *merge_bases_many(struct commit *one, int n, struct co
 		}
 	}
 
-	/* Clean up the result to remove stale ones */
 	free_commit_list(list);
-	list = result; result = NULL;
+	return result;
+}
+
+static struct commit_list *merge_bases_many(struct commit *one, int n, struct commit **twos)
+{
+	struct commit_list *list = NULL;
+	struct commit_list *result = NULL;
+	int i;
+
+	for (i = 0; i < n; i++) {
+		if (one == twos[i])
+			/*
+			 * We do not mark this even with RESULT so we do not
+			 * have to clean it up.
+			 */
+			return commit_list_insert(one, &result);
+	}
+
+	if (parse_commit(one))
+		return NULL;
+	for (i = 0; i < n; i++) {
+		if (parse_commit(twos[i]))
+			return NULL;
+	}
+
+	list = paint_down_to_common(one, n, twos);
+
 	while (list) {
 		struct commit_list *next = list->next;
 		if (!(list->item->object.flags & STALE))