R1:7ef236c9a647

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…