Fix a recent regression to "git rebase -i" and add tests that would
have caught it and others.

* pw/rebase-i-regression-fix-tests:
  t3420: fix under GETTEXT_POISON build
  rebase: add more regression tests for console output
  rebase: add regression tests for console output
  rebase -i: add test for reflog message
  sequencer: print autostash messages to stderr