builtin/repack: fix leaking line buffer when packing promisors

In `repack_promisor_objects()` we read output from git-pack-objects(1)
line by line, using `strbuf_getline_lf()`. We never free the line
buffer, causing a memory leak. Plug it.

This leak is being hit in t5616, but plugging it alone is not
sufficient to make the whole test suite leak free.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -425,9 +425,11 @@ static void repack_promisor_objects(const struct pack_objects_args *args,
 
 		free(promisor_name);
 	}
+
 	fclose(out);
 	if (finish_command(&cmd))
 		die(_("could not finish pack-objects to repack promisor objects"));
+	strbuf_release(&line);
 }
 
 struct pack_geometry {