[FIXED JENKINS-3907] Let all runs of a matrix build update to the same Mercurial revision

This commit fixes [JENKINS-3907](https://issues.jenkins-ci.org/browse/JENKINS-3907), so that all runs of a matrix build update to the same Mercurial revision.

I've got push access to this repository, so I can merge in this pull request myself, but I figured I should let some other maintainers take a look at this before I do so.