fetch-pack: clear pack lockfiles list

fetch-pack: clear pack lockfiles list

If the --lock-pack option is passed (which it typically is when
fetch-pack is used under the hood by smart-http), then we may end up
with entries in our pack_lockfiles string_list. We need to clear them
before returning to avoid a leak.

In git-fetch this isn't a problem, since the same cleanup happens via
transport_unlock_pack(). But the leak is detectable in t5551, which does
http fetches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>