bisect: use util.system and fix good/bad when using -c The existing scheme using util.find_exe and subprocess.call meant we couldn't use simple shell commands in tests. Fix that. Also, it mistakenly used status from the system() call rather than good from the bisect call in reporting results.

bisect: limit considered set to descendants of first good rev

tests: add -k to test scripts matching keywords argument is a space-separated list of keywords that are searched for in the name and body of each test. This makes it easy to run only tests related to tags, hgweb, revert, etc. (eg -k "tag hgweb revert").