I know this flag is a little strange, but internally, the implementation of diff drivers is such that the driver can add in diff options on a per-file basis (for the files that that driver applies to). If the driver has `diff.<driver>.binary` set to false, then it adds in the existing `GIT_DIFF_FORCE_TEXT` flag. If it has that value set to true, it was convenient for me to have a `GIT_DIFF_FORCE_BINARY` that was parallel. I'm not certain if anyone will actively choose to use this flag, but it doesn't seem like it will hurt.