reflog: clear leftovers in reflog_expiry_cleanup()

reflog: clear leftovers in reflog_expiry_cleanup()

reflog_expiry_prepare() calls mark_reachable(), which recurively flags
commits as REACHABLE.  The traversal stops beyond a certain age
threshold; the boundary commits also marked as REACHABLE and put back
into mark_list at the end.  unreachable() finishes the traversal down to
the roots if necessary -- but if all interesting commits are younger
than the age threshold then only recent commits need to be visited.

When this optimization works then the boundary commits still sit there
in mark_list at the end.  Clear their REACHABLE flag and release the
commit list allocations.

While at it remove a duplicate code line from mark_reachable(); the same
flag is already set five lines up.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>