packfile: introduce macro to iterate through packs

packfile: introduce macro to iterate through packs

We have a bunch of different sites that want to iterate through all
packs of a given `struct packfile_store`. This pattern is somewhat
verbose and repetitive, which makes it somewhat cumbersome.

Introduce a new macro `repo_for_each_pack()` that removes some of the
boilerplate.

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