Merge branch 'js/prepare-sequencer'

Merge branch 'js/prepare-sequencer'

Update of the sequencer codebase to make it reusable to reimplement
"rebase -i" continues.

* js/prepare-sequencer: (27 commits)
  sequencer: mark all error messages for translation
  sequencer: start error messages consistently with lower case
  sequencer: quote filenames in error messages
  sequencer: mark action_name() for translation
  sequencer: remove overzealous assumption in rebase -i mode
  sequencer: teach write_message() to append an optional LF
  sequencer: refactor write_message() to take a pointer/length
  sequencer: roll back lock file if write_message() failed
  sequencer: stop releasing the strbuf in write_message()
  sequencer: left-trim lines read from the script
  sequencer: support cleaning up commit messages
  sequencer: support amending commits
  sequencer: allow editing the commit message on a case-by-case basis
  sequencer: introduce a helper to read files written by scripts
  sequencer: prepare for rebase -i's commit functionality
  sequencer: remember the onelines when parsing the todo file
  sequencer: get rid of the subcommand field
  sequencer: avoid completely different messages for different actions
  sequencer: strip CR from the todo script
  sequencer: completely revamp the "todo" script parsing
  ...