Rename expire_reflog_cb to expire_reflog_policy_cb

Rename expire_reflog_cb to expire_reflog_policy_cb

This is the first step towards separating the data needed by the
policy code from the data needed by the reflog expiration machinery.

(In a moment we will add a *new* "struct expire_reflog_cb" for the use
of expire_reflog() itself, then move fields selectively from
expire_reflog_policy_cb to expire_reflog_cb.)

Signed-off-by: Michael Haggerty <redacted>
Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>