Add diff option to determine which blobs are binary.

When Git diffs a filepair, if either side is binary it outputs:

Binary files a/foo and b/foo differ

From this we can infer that one or both of the files are binary, but we can't be sure which ones.

GitLab Rails needs to known exactly which blobs in a diff filepair are binary when fetching blob contents to avoid fetching binary blobs.

If the diff output for accurately specified which specific blobs of the filepair were binary, we could avoid this extra check it Rails.