Merge branch 'jc/fsck-nul-in-commit'

Merge branch 'jc/fsck-nul-in-commit'

"git fsck" learned to catch NUL byte in a commit object as
potential error and warn.

* jc/fsck-nul-in-commit:
  fsck: detect and warn a commit with embedded NUL
  fsck_commit_buffer(): do not special case the last validation