Merge branch 'ml/filter-branch-no-op-error'

"git filter-branch" learned to use a different exit code to allow
the callers to tell the case where there was no new commits to
rewrite from other error cases.

* ml/filter-branch-no-op-error:
  filter-branch: return 2 when nothing to rewrite

diff --cc git-filter-branch.sh
Simple merge