write_remote_refs(): create packed (rather than extra) refs

write_remote_refs(): create packed (rather than extra) refs

write_remote_refs() creates new packed refs from references obtained
from the remote repository, which is "out of thin air" as far as the
local repository is concerned.  Previously it did this by creating
"extra" refs, then calling pack_refs() to bake them into the
packed-refs file.  Instead, create packed refs (in the packed
reference cache) directly, then call pack_refs().

Aside from being more logical, this is another step towards removing
extra refs entirely.

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