[m-rev.] diff: replace more uses of arity

[Zoltan Somogyi]

This should not need review.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.ar2
Type: application/octet-stream
Size: 519 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20220218/c2016d11/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.ar2
Type: application/octet-stream
Size: 7971 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20220218/c2016d11/attachment-0001.obj>