Adjust DiffFilesWithTolerance to help poor cygwin's mmap facility by

handling zero length files a little more intelligently. If both files are
zero length then we return 0 (true) indicating a match. If only one of the
files is zero length then we return 1 (false) indicating that the files
differ. If the files don't agree in length then they can't match so we
skip the first loop that looks for a quick match.

llvm-svn: 20200