[m-rev.] diff: fix misleading predicate names and comments

[Zoltan Somogyi]

This should make future work on this module easier.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.dir
Type: application/octet-stream
Size: 1437 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20190410/51ce8634/attachment-0002.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.dir
Type: application/octet-stream
Size: 46102 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20190410/51ce8634/attachment-0003.obj>