paint_down_to_common(): parse commit before relying on its timestamp

When refactoring the merge-base computation to reduce the pairwise
O(n*(n-1)) traversals to parallel O(n) traversals, the code forgot
that timestamp based heuristics needs each commit to have been
parsed.  This caused an empty "git pull" to spend cycles, traversing
the history all the way down to 0 (because an unparsed commit object
has 0 timestamp, and any other commit object with positive timestamp
will be processed for its parents, all getting parsed), only to come
up with a merge message to be used.

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

commit.c

@@ -581,6 +581,7 @@ static struct commit *interesting(struct commit_list *list)
 	return NULL;
 }
 
+/* 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 commit_list *list = NULL;
@@ -589,6 +590,8 @@ static struct commit_list *paint_down_to_common(struct commit *one, int n, struc
 
 	one->object.flags |= PARENT1;
 	commit_list_insert_by_date(one, &list);
+	if (!n)
+		return list;
 	for (i = 0; i < n; i++) {
 		twos[i]->object.flags |= PARENT2;
 		commit_list_insert_by_date(twos[i], &list);
@@ -709,6 +712,8 @@ static int remove_redundant(struct commit **array, int cnt)
 	redundant = xcalloc(cnt, 1);
 	filled_index = xmalloc(sizeof(*filled_index) * (cnt - 1));
 
+	for (i = 0; i < cnt; i++)
+		parse_commit(array[i]);
 	for (i = 0; i < cnt; i++) {
 		struct commit_list *common;