Fix normalize fail on non-lowercase filename (Issue1273) regression introduced in 1e2850ed8171

fix changegroup.notify hook broken by fb42030d79d6

dirstate: explain why appending instead of os.path.join() is safe

performance: normalize self._root, avoid calling os.path.join() in dirstate In dirstate, self._join() might get called a lot. Instead of calling it we create self._rootdir and we then only need to append the filename.