mq: changed help for qpop -f to reflect actual behaviour

convert/svn: better error when hg cannot call itself (issue1838)

patch: handle symlink updates/replacements (issue1785)

patch: handle symlinks without symlinkhunk Symlink creations and deletions were handled with a special symlinkhunk object, working like a binary hunk. However, this model does not support symlink updates or replacements, so we teach regular hunks how to handle symlinks.

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").