subrepo: encode unicode path names (issue3610)
Subversion 1.7 changes its XML output to include an explicit encoding tag:
<?xml version="1.0" encoding="UTF-8"?>
This triggers xml.dom.minidom to always return unicode strings, causing
other parts of the code to explode.
We unconditionally encode path names before handing them back, which
works with both str (actually a no-op) and unicode values.