dirstate._walkexplicit: don't bother normalizing '.' The overwhelmingly common case is running commands like 'hg diff' with no arguments. Therefore the only file that'll be listed is the root directory. Normalizing that's just a waste of time. This means that for a plain 'hg diff' we'll never need to construct the foldmap, saving us a significant chunk of time. On case-insensitive HFS+ on OS X, for a large repository with over 200,000 files, this brings down 'hg diff' from 2.97 seconds to 2.36.