git-svn: fix URL canonicalization during init w/ SVN 1.7+

git-svn: fix URL canonicalization during init w/ SVN 1.7+

URL canonicalization when full URLs are passed became broken
when using SVN::_Core::svn_dirent_canonicalize under SVN 1.7.

Ensure we canonicalize paths and URLs with appropriate functions
for each type from now on as the path/URL-agnostic
SVN::_Core::svn_path_canonicalize function is deprecated in SVN.

Tested with the following commands:

  git svn init -T svn://svn.code.sf.net/p/squirrelmail/code/trunk
  git svn init -b svn://svn.code.sf.net/p/squirrelmail/code/branches

Reported-by: Adam Dinwoodie <adam@dinwoodie.org>
  http://mid.gmane.org/20160315162344.GM29016@dinwoodie.org
Signed-off-by: Eric Wong <normalperson@yhbt.net>