GitPoller support for multiple branches per repo

When trying to get two GitPoller's working with the same repourl but with different branches, master crashes with this error:

File "/home/buildbot/lib/python2.6/site-packages/twisted/application/service.py", line 303, in addService
            " '%s'" % service.name)
        exceptions.RuntimeError: cannot have two services with same name ...

The fix is to combine repourl and branch to get the name of the service.