submodule: fix leaking seen submodule names

We keep track of submodules we have already seen via a string map such
that we don't process the same submodule twice. We never free that map
though, causing a memory leak.

Fix this leak by clearing the map.

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

submodule.c

@@ -1880,6 +1880,7 @@ int fetch_submodules(struct repository *r,
 	strvec_clear(&spf.args);
 out:
 	free_submodules_data(&spf.changed_submodule_names);
+	string_list_clear(&spf.seen_submodule_names, 0);
 	return spf.result;
 }