t5500: give fully-qualified refs to fetch-pack

t5500: give fully-qualified refs to fetch-pack

The fetch-pack documentation is very clear that refs given
on the command line are to be full refs:

  <refs>...::
          The remote heads to update from. This is relative to
          $GIT_DIR (e.g. "HEAD", "refs/heads/master").  When
          unspecified, update from all heads the remote side has.

and this has been the case since fetch-pack was originally documented in
8b3d9dc ([PATCH] Documentation: clone/fetch/upload., 2005-07-14).

Let's follow our own documentation to set a good example,
and to avoid breaking when this restriction is enforced in
the next patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>