reachable.c: extract `obj_is_recent()`

reachable.c: extract `obj_is_recent()`

When enumerating objects in order to add recent ones (i.e. those whose
mtime is strictly newer than the cutoff) as tips of a reachability
traversal, `add_recent_object()` discards objects which do not meet the
recency criteria.

The subsequent commit will make checking whether or not an object is
recent also consult the list of hooks in `pack.recentHook`. Isolate this
check in its own function to keep the additional complexity outside of
`add_recent_object()`.

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