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.

diff: use fctx.size() to test empty fctx.size() could have a fast path that does not require loading content.

diff: use fctx.isbinary() to test binary The end goal is to avoid calling fctx.data() when unnecessary. For example, if diff.nobinary=1 and files are binary, the expected behavior is to print "Binary file has changed". That could avoid reading fctx.data() sometimes. This is mainly to enable an external LFS extension to skip expensive binary file loading sometimes (read: most of the time with diff.nobinary=1 and diff.text=0), without any behavior changes to mercurial (i.e. whether a file is LFS or not does not change any behavior, LFS could be 100% transparent to users).