git-svn: don't rely on do_switch + reparenting with svn(+ssh)://

git-svn: don't rely on do_switch + reparenting with svn(+ssh)://

I can't seem to figure out what I or the SVN libraries are doing
wrong, but it appears to be related to reparent and probably
some global structure that gets reset if multiple SVN
connections are being used.

So now, in order to use do_switch; we'll open a new connection
to the repository with the complete URL; but we can't seem to
ever use an existing Ra object after another one has been
created...

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