Add a new SubtreeSymbolTransform.
Patch by: Jon Foster <jon.foster@cabot.co.uk>
This wraps another SymbolTransform and limits it to only apply within
a certain directory; e.g.,
SubtreeSymbolTransform('myrepo/some_module',
RegexpSymbolTransform('.*-br', r'\1-branch'))
will only do the regexp transform within the specified module.
This allows more powerful transforms than the existing
SubtreeSymbolMapper.
git-svn-id: http://cvs2svn.tigris.org/svn/cvs2svn/trunk@5079 be7e6eca-30d4-0310-a8e5-ac0d63af7087