parse_diff (Spec::Runner::Options)

    # File lib/spec/runner/options.rb, line 199
          def parse_diff(format)
            case format
            when :context, 'context', 'c'
              @diff_format  = :context
              default_differ
            when :unified, 'unified', 'u', '', nil
              @diff_format  = :unified
              default_differ
            else
              @diff_format  = :custom
              self.differ_class = load_class(format, 'differ', '--diff')
            end
          end