An early part of piece-by-piece rewrite of "git bisect".
* pb/bisect-helper:
bisect--helper: `is_expected_rev` & `check_expected_revs` shell function in C
t6030: explicitly test for bisection cleanup
bisect--helper: `bisect_clean_state` shell function in C
bisect--helper: `write_terms` shell function in C
bisect--helper: rewrite `check_term_format` shell function in C
bisect--helper: use OPT_CMDMODE instead of OPT_BOOL