diff: add a fast path to avoid loading binary contents When diffing binary contents, with certain configs, we can show "Binary file <name> has changed" without actual content. That allows a fast path where we could avoid providing actual binary contents. Note: in that case we still need to test if two contents are the same, that's done by using "filectx.cmp", which could have its own fast path.

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

diff: draw a table about binary diff behaviors The table should make it easier to reason about future changes.