Don't bother passing ref log details to walker in builtin-fetch

Don't bother passing ref log details to walker in builtin-fetch

When using the walker API within builtin-fetch we don't allow
it to update refs locally; instead that action is reserved for
builtin-fetch's own main loop once the objects have actually
been downloaded.

Passing NULL here will bypass the unnecessary malloc/free of a
string buffer within the walker API.  That buffer is never used
because the prior argument (the refs to update) is also NULL.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>