pretty: give placeholders to reflog identity

pretty: give placeholders to reflog identity

When doing a reflog walk, you can get some information about
the reflog (such as the subject line), but not the identity
information (i.e., name and email).

Let's make those available, mimicing the options for author
and committer identity.

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