Switch the Mercurial support to use the command line tool.
Our Mercurial support interfaced with the 'hg' module to cat files in a

repository, but this can easily be accomplished by the command line

tool. That tool is the correct API to use, not the module. More

importantly, it resolves a licensing issue due to the 'hg' module being

GPL, which was a terrible oversight that we want to address. This will

be backported to prior releases.

Unit tests pass.