send-pack: track errors for each ref

send-pack: track errors for each ref

Instead of keeping the 'ret' variable, we instead have a
status flag for each ref that tracks what happened to it.
We then print the ref status after all of the refs have
been examined.

This paves the way for three improvements:
  - updating tracking refs only for non-error refs
  - incorporating remote rejection into the printed status
  - printing errors in a different order than we processed
    (e.g., consolidating non-ff errors near the end with
    a special message)

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Alex Riesen <raa.lkml@gmail.com>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>