git-svn: --follow-parent works with svn-remotes multiple branches

git-svn: --follow-parent works with svn-remotes multiple branches

Bugs fixed:

 * We didn't allow manually (not using git-svn) init-ed
   remotes/fetch refspecs to be used before.  It works now
   because that's what I did in this test.  git-svn init should
   offer more control in the future.
 * correctly strip paths in the delta editor when using
   do_switch().
 * Make the -i / GIT_SVN_ID option work correctly when doing
   fetch on a multi-ref svn-remote

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