expire_reflog(): treat the policy callback data as opaque

expire_reflog(): treat the policy callback data as opaque

Now that expire_reflog() doesn't actually look in the
expire_reflog_policy_cb data structure, we can make it opaque:

* Change the callers of expire_reflog() to pass it a pointer to an
  entire "struct expire_reflog_policy_cb" rather than a pointer to a
  "struct cmd_reflog_expire_cb".

* Change expire_reflog() to accept the argument as a "void *" and
  simply pass it through to the policy functions.

* Change the policy functions, reflog_expiry_prepare(),
  reflog_expiry_cleanup(), and should_expire_reflog_ent(), to accept
  "void *cb_data" arguments and cast them back to "struct
  expire_reflog_policy_cb" internally.

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>