reachable: report precise timestamps from objects in cruft packs

When generating a cruft pack, the caller within pack-objects will want
to know the precise timestamps of cruft objects (i.e., their
corresponding values in the .mtimes table) rather than the mtime of the
cruft pack itself.

Teach add_recent_packed() to lookup each object's precise mtime from the
.mtimes file if one exists (indicated by the is_cruft bit on the
packed_git structure).

A couple of small things worth noting here:

- load_pack_mtimes() needs to be called before asking for
nth_packed_mtime(), and that call is done lazily here. That function
exits early if the .mtimes file has already been opened and parsed,
so only the first call is slow.

- Checking the is_cruft bit can be done without any extra work on the
caller's behalf, since it is set up for us automatically as a
side-effect of calling add_packed_git() (just like the 'pack_keep'
and 'pack_promisor' bits).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>