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.

performance: do not stat() things when not required Ignore unknown files if we don't need them (eg in hg diff). It slows things down a little bit for big trees (kernel repo), since _join() is called for each file instead of for each directory. fix issue567

Merge with crew-stable