bisect: fix memory leak when returning best element

When `find_bisection()` returns a single list entry, it leaks the other
entries. Move the to-be-returned item to the front and free the
remainder.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -399,8 +399,12 @@ void find_bisection(struct commit_list **commit_list, int *reaches,
 	/* Do the real work of finding bisection commit. */
 	best = do_find_bisection(list, nr, weights, find_all);
 	if (best) {
-		if (!find_all)
+		if (!find_all) {
+			list->item = best->item;
+			free_commit_list(list->next);
+			best = list;
 			best->next = NULL;
+		}
 		*reaches = weight(best);
 	}
 	free(weights);