[m-rev.] diff: compute dependencies of .opt files just once

[Zoltan Somogyi]

This should not need review.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.opt
Type: application/octet-stream
Size: 1033 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20200528/750caf0b/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.opt
Type: application/octet-stream
Size: 11430 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20200528/750caf0b/attachment-0001.obj>