reflog delete: narrow scope of "cmd" passed to count_reflog_ent()

reflog delete: narrow scope of "cmd" passed to count_reflog_ent()

Change the "cb_data" we pass to the count_reflog_ent() to be the
&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-loop that uses &cb, except for
the "cmd" member.

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