pack_one_ref(): use write_packed_entry() to do the writing

pack_one_ref(): use write_packed_entry() to do the writing

Change pack_refs() to work with a file descriptor instead of a FILE*
(making the file-locking code less awkward) and use
write_packed_entry() to do the writing.

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