git-svn: path canonicalization uses SVN API

git-svn: path canonicalization uses SVN API

All tests pass with SVN 1.6.  SVN 1.7 remains broken, not worrying
about it yet.

SVN changed its path canonicalization API between 1.6 and 1.7.
http://svnbook.red-bean.com/en/1.6/svn.developer.usingapi.html#svn.developer.usingapi.urlpath
http://svnbook.red-bean.com/en/1.7/svn.developer.usingapi.html#svn.developer.usingapi.urlpath

The SVN API does not accept foo/.. but it also doesn't canonicalize
it.  We have to do it ourselves.

[ew: commit title, fall back if SVN <= 1.6 fails to canonicalize]

Signed-off-by: Eric Wong <normalperson@yhbt.net>