Merge branch 'cm/rebase-i-updates'

Merge branch 'cm/rebase-i-updates'

Follow-up fixes to "cm/rebase-i" topic.

* cm/rebase-i-updates:
  doc/rebase -i: fix typo in the documentation of 'fixup' command
  t/t3437: fixup the test 'multiple fixup -c opens editor once'
  t/t3437: use named commits in the tests
  t/t3437: simplify and document the test helpers
  t/t3437: check the author date of fixed up commit
  t/t3437: remove the dependency of 'expected-message' file from tests
  t/t3437: fixup here-docs in the 'setup' test
  t/lib-rebase: update the documentation of FAKE_LINES
  rebase -i: clarify and fix 'fixup -c' rebase-todo help
  sequencer: rename a few functions
  sequencer: fixup the datatype of the 'flag' argument