bisect: document run behavior with exit codes 126 and 127
Shells report non-executable and missing commands with exit codes 126
and 127, respectively. For historical reasons "git bisect run"
interprets them as indicating a bad commit, though. Document the
current behavior by adding basic tests that cover these cases.
Reported-by: Ramkumar Ramachandra <r@artagnon.com> Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>