diff: determine DIFFABLE-ness for binaries

diff: determine DIFFABLE-ness for binaries

Always set `GIT_DIFF_PATCH_DIFFABLE` for all files, regardless of
binary-ness, so that the binary callback is invoked to either
show the binary contents, or just print the standard "Binary files
differ" message.  We may need to do deeper inspection for binary
files where we have avoided loading the contents into a file map.