Merge branch 'en/rebase-incompatible-opts'

"git rebase" often ignored incompatible options instead of
complaining, which has been corrected.

* en/rebase-incompatible-opts:
  rebase: provide better error message for apply options vs. merge config
  rebase: put rebase_options initialization in single place
  rebase: fix formatting of rebase --reapply-cherry-picks option in docs
  rebase: clarify the OPT_CMDMODE incompatibilities
  rebase: add coverage of other incompatible options
  rebase: fix incompatiblity checks for --[no-]reapply-cherry-picks
  rebase: fix docs about incompatibilities with --root
  rebase: remove --allow-empty-message from incompatible opts
  rebase: flag --apply and --merge as incompatible
  rebase: mark --update-refs as requiring the merge backend