Mercurial 2.3 breaks hgsubversion

https://answers.atlassian.com/questions/93171/does-sourcetree-1-5-4-on-a-mac-work-with-system-mercurial-version-2-3-1

Turns out they've changed something in hg 2.3 which breaks the hgsubversion extension and makes most hg commands fail.

Workaround: disable hgsubversion

Fix: probably need to update the embedded hgsubversion extension, but will need re-testing.