R1:1e3c9bdfee76

Update Diff: fix dead-end when not yours

Summary:
Fix "Update Patch" for generic Differential revisions.

You can now help a coworker to update their patch, from the web.

So, if you visit Diff 111 (not yours) and you click "Update Diff", and you paste a valid diff,
you arrive here:

https://we.phorge.it/differential/diff/222/?revisionID=111

That page as default was suggesting some Diffs that are yours. Plus, now it shows Diff 111.

Omitting the Diff from which the workflow was started made no sense…