Merge branch 'mr/bisect-in-c-1'

Merge branch 'mr/bisect-in-c-1'

Underlying machinery of "git bisect--helper" is being refactored
into pieces that are more easily reused.

* mr/bisect-in-c-1:
  bisect: libify `bisect_next_all`
  bisect: libify `handle_bad_merge_base` and its dependents
  bisect: libify `check_good_are_ancestors_of_bad` and its dependents
  bisect: libify `check_merge_bases` and its dependents
  bisect: libify `bisect_checkout`
  bisect: libify `exit_if_skipped_commits` to `error_if_skipped*` and its dependents
  bisect--helper: return error codes from `cmd_bisect__helper()`
  bisect: add enum to represent bisect returning codes
  bisect--helper: introduce new `decide_next()` function
  bisect: use the standard 'if (!var)' way to check for 0
  bisect--helper: change `retval` to `res`
  bisect--helper: convert `vocab_*` char pointers to char arrays