Add an "Onto Branch" selector control to "Land Revision" dialog

Summary:
Ref T9952. This adds a typeahead so you can pick a branch to target.

It does not choose a default branch, the user must pick a branch explicitly.

Test Plan:

• Landed rGITTESTd587fada48fc to master (by typing "master").
• Landed rGITTEST86c339b2ef01 to notmaster (by typing "notmaster").

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9952

Differential Revision: https://secure.phabricator.com/D14733