[m-rev.] diff: factor out some common code

[Zoltan Somogyi]

This should not need review.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.tsubn
Type: application/octet-stream
Size: 535 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20201013/2002d350/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.tsubn
Type: application/octet-stream
Size: 10475 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20201013/2002d350/attachment-0001.obj>