Merge branch 'en/rebase-merge-on-sequencer'

Merge branch 'en/rebase-merge-on-sequencer'

"git rebase --merge" as been reimplemented by reusing the internal
machinery used for "git rebase -i".

* en/rebase-merge-on-sequencer:
  rebase: implement --merge via the interactive machinery
  rebase: define linearization ordering and enforce it
  git-legacy-rebase: simplify unnecessary triply-nested if
  git-rebase, sequencer: extend --quiet option for the interactive machinery
  am, rebase--merge: do not overlook --skip'ed commits with post-rewrite
  t5407: add a test demonstrating how interactive handles --skip differently
  rebase: fix incompatible options error message
  rebase: make builtin and legacy script error messages the same