packfile: split up responsibilities of `reprepare_packed_git()`

packfile: split up responsibilities of `reprepare_packed_git()`

In `reprepare_packed_git()` we perform a couple of operations:

  - We reload alternate object directories.

  - We clear the loose object cache.

  - We reprepare packfiles.

While the logic is hosted in "packfile.c", it clearly reaches into other
subsystems that aren't related to packfiles.

Split up the responsibility and introduce `odb_reprepare()` which now
becomes responsible for repreparing the whole object database. The
existing `reprepare_packed_git()` function is refactored accordingly and
only cares about reloading the packfile store now.

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