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