R1:954cd4b1b6e7

When `arc` has sent target branch data up after D14736, use it in the UI and "Land Revision"

Summary:
Ref T9952. Ref T3462. After D14736, if we have information about the target/"onto" branch, use it in the UI:

- Show "feature (branched from master)" instead of "feature".
- Default "Land Revision" to hit the correct branch.

Test Plan:
- Branched from `test` with branch tracking.
- Diffed.
- Saw "feature (branched from test)" in UI.
- Saw "test" fill as default in "Land Revision", despite the…