Rework the Mercurial version checking to not break.

The Mercurial version checking was breaking in weird ways. This change
attempts to do a better job at checking the version.

First, we use their own version() function, if available.

If we can't import that, we'll try getting __version__ again, but in a
different way. We'll import the module and not the constant within it.

If that doesn't work, or if we get a version of None, we set it to an
empty string, which will result in the 'if <= 1.2' check passing.

Unit tests pass.