Attach Diff to Revision: fix Repository asked twice

Summary:
If you manually upload a Diff (patch) from the Differential web
interface you can select the Repository.

We fix the default value of the Repository field during creation mode,
guessing the repo from the potentially already-available diff.

Closes T15537

Test Plan:

• Create a cute Differential Revision from web
• Pick a Repository > Continue
• Create a new Revision > Continue

At this point you should still have that repository instead of nothing.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15537

Differential Revision: https://we.phorge.it/D25338