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: NJunio C Hamano <gitster@pobox.com>

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

t/t7600-merge.sh

@@ -490,4 +490,26 @@ test_expect_success 'merge c1 with c0, c2, c0, and c1' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge c1 with c0, c2, c0, and c1' '
+       git reset --hard c1 &&
+       git config branch.master.mergeoptions "" &&
+       test_tick &&
+       git merge c0 c2 c0 c1 &&
+       verify_merge file result.1-5 &&
+       verify_parents $c1 $c2
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c1 with c1 and c2' '
+       git reset --hard c1 &&
+       git config branch.master.mergeoptions "" &&
+       test_tick &&
+       git merge c1 c2 &&
+       verify_merge file result.1-5 &&
+       verify_parents $c1 $c2
+'
+
+test_debug 'gitk --all'
+
 test_done