Move MercurialClient class to be in the same part of the post-review source as the other SCMClient subclasses.  It took me a moment to find the Mercurial class since it is currently distant from its peers and I'd like to save other people from spending that moment.  :)

I don't use Mercurial, so I don't have a good way to test that class, but I confirmed that the post-review script still runs after this change.