When using "Update Diff" from the web UI, prefill "Repository" properly

Summary: Ref T9499. When using the manual "Update Diff" workflow on the web, the "Repository" field isn't pre-filled properly. This can lead to revisions losing their repository after a manual update.

Test Plan: Did a manual update of a revision with a repository, saw it stick.

Maniphest Tasks: T9499

Differential Revision: https://secure.phabricator.com/D21638