odb: introduce mtime fields for object info requests

odb: introduce mtime fields for object info requests

There are some use cases where we need to figure out the mtime for
objects. Most importantly, this is the case when we want to prune
unreachable objects. But getting at that data requires users to manually
derive the info either via the loose object's mtime, the packfiles'
mtime or via the ".mtimes" file.

Introduce a new `struct object_info::mtimep` pointer that allows callers
to request an object's mtime. This new field will be used in a
subsequent commit.

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