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