pack_if_possible_fn(): use ref_type() instead of is_per_worktree_ref()

pack_if_possible_fn(): use ref_type() instead of is_per_worktree_ref()

is_per_worktree_ref() will soon be made private, so use the public
interface, ref_type(), in its place. And now that we're using
ref_type(), we can make it clear that we won't pack pseudorefs. This was
the case before, but due to the not-so-obvious reason that this function
is applied to references via the loose reference cache, which only
includes references that live inside "refs/".

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>