git-svn: Move all git-svn-related paths into $GIT_DIR/svn

Since GIT_SVN_ID usage is probably going to become more
widespread <evil grin>, we won't run the chance of somebody
having a GIT_SVN_ID name that conflicts with one of the default
directories that already exist in $GIT_DIR (branches/tags).

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