reduce_heads(): protect from duplicate input

Because we do not try computing merge base with itself for obvious
reasons, the code was not prepared for an arguably insane case of
the caller feeding the same commit twice to it.

Noticed and test written by Sverre Hvammen Johansen

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

1 files changed, 9 insertions, 2 deletions

diff --git a/commit.c b/commit.c
index 03e73f323..5148ec552 100644
--- a/commit.c
+++ b/commit.c
@@ -745,15 +745,22 @@ struct commit_list *reduce_heads(struct commit_list *heads)
 	for (p = heads; p; p = p->next) {
 		struct commit_list *q, *base;
 
+		/* Do we already have this in the result? */
+		for (q = result; q; q = q->next)
+			if (p->item == q->item)
+				break;
+		if (q)
+			continue;
+
 		num_other = 0;
 		for (q = heads; q; q = q->next) {
 			if (p->item == q->item)
 				continue;
 			other[num_other++] = q->item;
 		}
-		if (num_other) {
+		if (num_other)
 			base = get_merge_bases_many(p->item, num_other, other, 1);
-		} else
+		else
 			base = NULL;
 		/*
 		 * If p->item does not have anything common with other