ref-filter: switch some uses of unsigned long to size_t

ref-filter: switch some uses of unsigned long to size_t

In the future, we'll want to pass some of the arguments of find_subpos
to strbuf_detach, which takes a size_t.  This is fine on systems where
that's the same size as unsigned long, but that isn't the case on all
systems.  Moreover, size_t makes sense since it's not possible to use a
buffer here that's larger than memory anyway.

Let's switch each use to size_t for these lengths in
grab_sub_body_contents and find_subpos.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>