expire_reflog(): pass flags through to expire_reflog_ent()

expire_reflog(): pass flags through to expire_reflog_ent()

Add a flags field to "struct expire_reflog_cb", and pass the flags
argument through to expire_reflog_ent(). In a moment we will start
using it to pass through flags that expire_reflog_ent() needs.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>