refs/files: sort reflogs returned by the reflog iterator

refs/files: sort reflogs returned by the reflog iterator

We use a directory iterator to return reflogs via the reflog iterator.
This iterator returns entries in the same order as readdir(3P) would and
will thus yield reflogs with no discernible order.

Set the new `DIR_ITERATOR_SORTED` flag that was introduced in the
preceding commit so that the order is deterministic. While the effect of
this can only been observed in a test tool, a subsequent commit will
start to expose this functionality to users via a new `git reflog list`
subcommand.

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