builtin/pack-objects: plug leaking list of keep-packs

The `--keep-pack` option of git-pack-objects(1) populates the arguments
into a string list. And while the list is marked as `NODUP` and thus
won't duplicate the strings, the list entries themselves still need to
be free'd. We don't though, causing a leak.

Plug it.

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

builtin/pack-objects.c

@@ -4641,6 +4641,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 cleanup:
 	clear_packing_data(&to_pack);
 	list_objects_filter_release(&filter_options);
+	string_list_clear(&keep_pack_list, 0);
 	strvec_clear(&rp);
 
 	return 0;