add ref/ and control/ subdirectories, so the control files and result
files do not clutter the directory, and it is clear which file has which
purpose.
isequal: New option --skip as a shortcut for --skipleft and --skipright
at once.
Use that for the .list-ref files, so they now contain the first line of
the output, too, which is ignored.
--binary: handled binary files
--empty: check if file is empty
--skipleft=<n>: Skip <n> lines at the start of the left (first) file
--skipright=<n>: Skip <n> lines at the start of the right (second) file
Note that --binary, --empty and one or both of --skipXXX are mutual
exclusive; that is, you cannot specify more than one of them at the same
time, or the behaviour will be unpredictable.
