receive-pack: drop "n/a" on unpacker errors

receive-pack: drop "n/a" on unpacker errors

The output from git push currently looks like this:

  $ git push dest HEAD
  fatal: [some message from index-pack]
  error: unpack failed: index-pack abnormal exit
  To dest
   ! [remote rejected] HEAD -> master (n/a (unpacker error))

That n/a is meant to be "the per-ref status is not
available" but the nested parentheses just make it look
ugly. Let's turn the final line into just:

   ! [remote rejected] HEAD -> master (unpacker error)

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