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