diff: correct binary testing logic This seems to be more correct given the table drawn in the previous patch. Namely, "losedatafn" and "opts.git" are removed, "not opts.text" is added. - losedatafn: diff output (binary) should not be affected by "losedatafn" - opts.git: binary testing is helpful for detecting a fast path in the next path. the fast path can also be used if opts.git is False - opts.text: if it's set, we should treat the content as non-binary