[PATCH v3 0/3] tracking per-ref errors on push

Here are cleaned-up versions of the previous patches I sent. The improvements are:

  1/3 send-pack: track errors for each ref
    - there was a t5404 regression because I removed the is_null_sha1()
      check before updating a tracking ref (but the replacement fix
      doesn't come until 2/3). Even though this check is not correct,
      it's better to fix it all at once correctly in 2/3.
    - clarified the desired git-push exit code in t5404
    - I renamed the struct elements to (hopefully) be a bit more obvious
    - readability cleanups (fixed some very long lines, hoisted some
      code into its own functions)

  2/3 send-pack: check ref->status before updating tracking refs
    - moved in fix from 1/3 mentioned above
    - add test for deleting tracking branches, which was broken in next
      but fixed by this patch

  3/3 send-pack: assign remote errors to each ref
    - squashed optimization patch
    - remove bogus parsing drawback in commit message
    - add test

I'm hoping to get feedback from the cc'd people:
  - Alex: please OK the modifications to t5404
  - Pierre: this should fix the tracking ref update issues you reported
  - Daniel: a general OK, since I am mangling your code :)

-Peff