Construct a new directory comparison object, to compare the
directories a and b. ignore is a list of names to
ignore, and defaults to ['RCS', 'CVS', 'tags']. hide is a
list of names to hide, and defaults to [os.curdir, os.pardir].
A dictionary mapping names in common_dirs to
dircmp objects.
Note that via __getattr__() hooks, all attributes are
computed lazilly, so there is no speed penalty if only those
attributes which are lightweight to compute are used.