path-list: fix path-list-insert return value

When path-list-insert is called on an existing path, it returned an
unrelated element in the list.  Luckily most of the callers are
ignoring the return value, but merge-recursive uses it at three places
and this would have resulted in a bogus rename detection.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

path-list.c

@@ -57,7 +57,7 @@ struct path_list_item *path_list_insert(const char *path, struct path_list *list
 	int index = add_entry(list, path);
 
 	if (index < 0)
-		index = 1 - index;
+		index = -1 - index;
 
 	return list->items + index;
 }