prune-packed: fix minor memory leak

We form all of our directories in a strbuf, but never release it.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/prune-packed.c

@@ -68,6 +68,7 @@ void prune_packed_objects(int opts)
 		rmdir(pathname.buf);
 	}
 	stop_progress(&progress);
+	strbuf_release(&pathname);
 }
 
 int cmd_prune_packed(int argc, const char **argv, const char *prefix)