compat: remove buggy monkey-patch of mercurial.hg.addbranchrevs()

The 'co' return value of the original function was never an integer. Therefore the condition was always true, causing 'co' to be None. This broke the case which was added as a test.

Presumedly, the 'co' used to be an integer at a hg version that we don’t support anymore. A comment said that the monkey-patch was added to defend against tracebacks. I was not able to come up with such a traceback. Since this was not tested, I can’t be sure, but there’s a chance that such a traceback is not possible anymore due to a change in hg.