dirstate: use util.normcase to build foldmap

merge with stable

largefiles: fix 'hg clone . ../foo' OSError abort Operating on a non-existant file can cause both IOError and OSError, depending on the function used: open raises IOError, os.lstat raises OSError. The largefiles code called dirstate.normal, which in turn calls os.lstat, so OSError is the right exception to catch here.