Merge branch 'js/sequencer-wo-die'

Merge branch 'js/sequencer-wo-die'

Lifts calls to exit(2) and die() higher in the callchain in
sequencer.c files so that more helper functions in it can be used
by callers that want to handle error conditions themselves.

* js/sequencer-wo-die:
  sequencer: ensure to release the lock when we could not read the index
  sequencer: lib'ify checkout_fast_forward()
  sequencer: lib'ify fast_forward_to()
  sequencer: lib'ify save_opts()
  sequencer: lib'ify save_todo()
  sequencer: lib'ify save_head()
  sequencer: lib'ify create_seq_dir()
  sequencer: lib'ify read_populate_opts()
  sequencer: lib'ify read_populate_todo()
  sequencer: lib'ify read_and_refresh_cache()
  sequencer: lib'ify prepare_revs()
  sequencer: lib'ify walk_revs_populate_todo()
  sequencer: lib'ify do_pick_commit()
  sequencer: lib'ify do_recursive_merge()
  sequencer: lib'ify write_message()
  sequencer: do not die() in do_pick_commit()
  sequencer: lib'ify sequencer_pick_revisions()