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.)
error: pretxncommit.nocommits hook failed: no commits allowed
transaction abort!
rollback completed
abort: no commits allowed
error: pretxncommit.nocommits hook failed: no commits allowed
transaction abort!
rollback completed
abort: no commits allowed