object-file: extract function to read object info from path

object-file: extract function to read object info from path

Extract a new function that allows us to read object info for a specific
loose object via a user-supplied path. This function will be used in a
subsequent commit.

Note that this also allows us to drop `stat_loose_object()`, which is
a simple wrapper around `odb_loose_path()` plus lstat(3p).

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