This is a re-roll of the series to make "git prune" in a corrupted
repository safer.

There are only minor tweaks from v1, but I think all of the raised
issues were addressed (there was discussion on some other points, but I
think they are OK as-is; more discussion is of course welcome).

The changes from v1 are:

  - use "bogus..name" as a bad refname instead of "bogus:name", as the
    latter has problems on Windows

  - fix broken &&-chains in tests

  - better commenting of missing-commit setup in t5312

  - typo-fixes in commit messages

Patches:

  [1/5]: t5312: test object deletion code paths in a corrupted repository
  [2/5]: refs: introduce a "ref paranoia" flag
  [3/5]: prune: turn on ref_paranoia flag
  [4/5]: repack: turn on "ref paranoia" when doing a destructive repack
  [5/5]: refs.c: drop curate_packed_refs

-Peff