Git::SVN{,::Ra}: canonicalize earlier

Git::SVN{,::Ra}: canonicalize earlier

This canonicalizes paths and urls as early as possible so we don't
have to remember to do it at the point of use.  It will fix a swath
of SVN 1.7 problems in one go.

Its ok to double canonicalize things.

SVN 1.7 still fails, still not worrying about that.

[ew: commit title]

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