Add symlink method to util.opener. Differences from os.symlink: - the symlink name is relative to the opener base directory - if a file with that name already exists, it's removed - if necessary, parent directories are created - if the system (OS or filesystem) doesn't support symlinks, a regular file is created. Its contents are the symlink target.

To run the tests, do:

cd tests/
python run-tests.py

See http://www.selenic.com/mercurial/wiki/index.cgi/WritingTests for
more information on writing tests.