Canonical SVN paths are not supposed to contain trailing slashes, but

we've never protected against this. PySVN happily passes our paths

through libsvn's path normalization functions, but subvertpy no longer

does this. This means that on Subvertpy 0.10 and higher, certain

operations will no longer work.
This change fixes our path normalization to always strip trailing

slashes, ensuring that we're more compliant with Subversion's path

requirements and fixing Subvertpy.

Unit tests pass on Subvertpy and PySVN.