subrepo: clean up lingering bytes/str issues in svn support
Path encoding is a little suspect in here, but it's not worse than it
was when we were on Python 2 only.
Subversion subrepo tests now pass in Python 3.
Differential Revision: https://phab.mercurial-scm.org/D5774
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273ce12ad8f1` will report some style violations on a very old commit).