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 repository having a different default branch.

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Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3462, T9952

Differential Revision: https://secure.phabricator.com/D14737