Differential - refine selecting a repository diff --> revision workflow

Summary: Fixes T6200. Ref T6237. When creating a diff from the web view, allow the user to select the repository at that time. When viewing a diff that has no associated revision and then creating a revision, pass along the repository phid to the create revision controller. Within the create revision controller, default the repository selector to this repository phid. Finally, in the editor, stop aggressively resetting the repository phid for every TYPE_UPDATE; rather, do so if its not a new object -- the diff should reign supreme in that case -- or if there's no repository -- let the diff be the guide.

Test Plan:
- made a diff with an associated repo, made a revision from the diff, saw the associated repo and it stuck on save!
- made a diff with an associated repo, made a revision from the diff but changed the repo and it stuck on save!
- made a diff with an associated repo, made a revision from the diff but changed the repo to nothing and it stuck on save!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6237, T6200

Differential Revision: https://secure.phabricator.com/D10872