reflog expire: narrow scope of "cb" in cmd_reflog_expire()

reflog expire: narrow scope of "cb" in cmd_reflog_expire()

As with the preceding change for "reflog delete", change the "cb_data"
we pass to callbacks to be &cb.cmd itself, instead of passing &cb and
having the callback lookup cb->cmd.

This makes it clear that the "cb" itself is the same memzero'd
structure on each iteration of the for-loops that use &cb, except for
the "cmd" member.

The "struct expire_reflog_policy_cb" we pass to reflog_expire() will
have the members that aren't "cmd" modified by the callbacks, but
before we invoke them everything except "cmd" is zero'd out.

This included the "tip_commit", "mark_list" and "tips". It might have
looked as though we were re-using those between iterations, but the
first thing we did in reflog_expiry_prepare() was to either NULL them,
or clobber them with another value.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>