OS X: try cheap ascii .lower() in normcase before making full unicode dance
This is similar to what is done in encoding.lower, introduced in c481761033bd.
This has been seen making 'hg up' and 'hg st' in a 50000+ files repo 13%
faster.
This might make Mercurial slightly slower for users who mainly use non-ASCII
filenames. That is a reasonable trade-off.