[PATCH v2 0/4] peel-ref optimization fixes

Here's a re-roll that takes into account the feedback from round 1:

  [1/4]: avoid segfaults on parse_object failure
  [2/4]: use parse_object_or_die instead of die("bad object")

These two patches are new; they are conceptually independent of the rest
of the series, but there's a textual dependency in later patches.

  [3/4]: pack-refs: write peeled entry for non-tags

Same as before, but rebased on patch 1, and s/``/$()/.

  [4/4]: pack-refs: add fully-peeled trait

Rewritten using Michael's approach, which is more readable.

-Peff