shallow: free local object_array allocations

The local object_array 'stack' in get_shallow_commits() function
does not free its dynamic elements before the function returns.
As a result elements remain allocated and their reference forgotten.

Also note, that test 'fetching deepen beyond merged branch' added by
'shallow: handling fetch relative-deepen' patch fails without this
correction in linux-leaks and linux-reftable-leaks test runs.

Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

shallow.c

@@ -198,6 +198,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 		}
 	}
 	deep_clear_commit_depth(&depths, free_depth_in_slab);
+	object_array_clear(&stack);
 
 	return result;
 }