Always remove trailing slashes from normalized SVN paths.
Review Request #9474 - Created Jan. 10, 2018 and submitted
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.