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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.