phabricator: follow-up phab auth improvements with backwards compat mode
We'll rip this out before we ship 4.6, but this gives people a window
to migrate.
Differential Revision: https://phab.mercurial-scm.org/D2703
py3: drop b'' from generate-working-copy-states.py output
I could make everything bytes, but decoding filename seemed easier and we
don't have to worry about non-ascii filenames here.