The Mercurial license is GPLv2

[Paul Moore]

2008/8/4 Germán Póo-Caamaño <gpoo at calcifer.org>:
> Certainly, they can run it in a separate process.  The thing is using
> the API to communicate with Mercurial instead of parsing the output.

So write a (GPL'd) extension to communicate with a persistent
Mercurial process via an easily parseable format (or XML, if you
prefer :-)). Best of both worlds. I assume that would be both in the
letter and the spirit (ie, Matt would be OK with it) of the license.

Paul.