Introduce for_each_recent_reflog_ent().

This can be used to scan only the last few kilobytes of a reflog, as a
cheap optimization when the data you are looking for is likely to be
found near the end of it.  The caller is expected to fall back to the
full scan if that is not the case.

Signed-off-by: Junio C Hamano <gitster@pobox.com>