More code in "git bisect" has been rewritten in C.

* tt/bisect-in-c:
  bisect--helper: `bisect_start` shell function partially in C
  bisect--helper: `get_terms` & `bisect_terms` shell function in C
  bisect--helper: `bisect_next_check` shell function in C
  bisect--helper: `check_and_set_terms` shell function in C
  wrapper: move is_empty_file() and rename it as is_empty_or_missing_file()
  bisect--helper: `bisect_write` shell function in C
  bisect--helper: `bisect_reset` shell function in C