Merge branch 'ab/bisect-cleanup'

Code clean-up.

* ab/bisect-cleanup:
  bisect: no longer try to clean up left-over `.git/head-name` files
  bisect: remove Cogito-related code
  bisect run: fix the error message
  bisect: verify that a bogus option won't try to start a bisection
  bisect--helper: make the order consistently `argc, argv`
  bisect--helper: simplify exit code computation