[PATCH 0/6] fix some parse_commit segfaults

This is a repost of the series here:

  http://thread.gmane.org/gmane.comp.version-control.git/235735/focus=235826

(the original was a single patch, followed by a 5-patch series, which I've combined here). It's mostly a cleanup, since parse_commit will only fail in corrupted repos, but I did run into it in a real (corrupted) repo.

  [1/6]: log_tree_diff: die when we fail to parse a commit
  [2/6]: assume parse_commit checks commit->object.parsed
  [3/6]: assume parse_commit checks for NULL commit
  [4/6]: use parse_commit_or_die instead of segfaulting
  [5/6]: use parse_commit_or_die instead of custom message
  [6/6]: checkout: do not die when leaving broken detached HEAD

-Peff