Merge branch 'rs/bisect-executable-not-found'

Merge branch 'rs/bisect-executable-not-found'

A not-so-common mistake is to write a script to feed "git bisect
run" without making it executable, in which case all tests will
exit with 126 or 127 error codes, even on revisions that are marked
as good.  Try to recognize this situation and stop iteration early.

* rs/bisect-executable-not-found:
  bisect--helper: double-check run command on exit code 126 and 127
  bisect: document run behavior with exit codes 126 and 127
  bisect--helper: release strbuf and strvec on run error
  bisect--helper: report actual bisect_state() argument on error