gc/repack: release packs when needed

gc/repack: release packs when needed

On Windows, files cannot be removed nor renamed if there are still
handles held by a process. To remedy that, we introduced the
close_all_packs() function.

Earlier, we made sure that the packs are released just before `git gc`
is spawned, in case that gc wants to remove no-longer needed packs.

But this developer forgot that gc itself also needs to let go of packs,
e.g. when consolidating all packs via the --aggressive option.

Likewise, `git repack -d` wants to delete obsolete packs and therefore
needs to close all pack handles, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>