reflog-walk: apply --since/--until to reflog dates

reflog-walk: apply --since/--until to reflog dates

When doing a reflog walk, we use the commit's date to
do any date limiting. In earlier versions of Git, this could
lead to nonsense results, since a skipped commit would
truncate the traversal. So a sequence like:

  git commit ...
  git checkout week-old-branch
  git checkout -
  git log -g --since=1.day.ago

would stop at the week-old-branch, even though the "git
commit" entry further back is still interesting.

As of the prior commit, which uses a parent-less traversal
of the reflog, you get the whole reflog minus any commits
whose dates do not match the specified options. This is
arguably useful, as you could scan the reflogs for commits
that originated in a certain range.

But more likely a user doing a reflog walk wants to limit
based on the reflog entries themselves. You can simulate
--until with:

  git log -g @{1.day.ago}

but there's no way to ask Git to traverse only back to a
certain date. E.g.:

  # show me reflog entries from the past day
  git log -g --since=1.day.ago

This patch teaches the revision machinery to prefer the
reflog entry dates to the commit dates when doing a reflog
walk. Technically this is a change in behavior that affects
plumbing, but the previous behavior was so buggy that it's
unlikely anyone was relying on it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>