Merge branch 'en/rebase-consistency'

Merge branch 'en/rebase-consistency'

"git rebase" behaved slightly differently depending on which one of
the three backends gets used; this has been documented and an
effort to make them more uniform has begun.

* en/rebase-consistency:
  git-rebase: make --allow-empty-message the default
  t3401: add directory rename testcases for rebase and am
  git-rebase.txt: document behavioral differences between modes
  directory-rename-detection.txt: technical docs on abilities and limitations
  git-rebase.txt: address confusion between --no-ff vs --force-rebase
  git-rebase: error out when incompatible options passed
  t3422: new testcases for checking when incompatible options passed
  git-rebase.sh: update help messages a bit
  git-rebase.txt: document incompatible options