Fix how setup.py identifies the Mercurial version. There is a problem with setup.py where it will not identify the Mercurial version properly when not being ran in within a repository even if mercurial/__version__.py exists. To fix, use mercurial.__version__.version when available before defaulting to "unknown". (Using mercurial.util.version() is not an option due to a dependency issue where osutil can be referenced before it is built.)