commit_packed_refs(): use a staging file separate from the lockfile

commit_packed_refs(): use a staging file separate from the lockfile

We will want to be able to hold the lockfile for `packed-refs` even
after we have activated the new values. So use a separate tempfile,
`packed-refs.new`, as a place to stage the new contents of the
`packed-refs` file. For now this is all done within
`commit_packed_refs()`, but that will change shortly.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>