py3: fix formatting of branchmap log messages with repo.filtername=None `"%s" % None` does not work on py3. I've extracted a little function for producing a formatted message given the filter name. Differential Revision: https://phab.mercurial-scm.org/D6644

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}