Default "Land Revision" dialog to land into the default branch for the repository

Summary:
Ref T9952. Default the branch target in the dialog to be whatever branch is the default branch for the repository.

This will be correct for repositories like ours (which land everything into master) and correct most of the time for repositories which have some other "primary" branch (maybe development).

It won't be great if there are multiple open lines of development in a repository (for example, some changes go to newdesignpro and some changes go to legacy-1.2). I'll do work in T3462 next to improve those cases so we can pick a better default.

Test Plan:

• Saw dialog default to "master".
• Changed repo default branch, saw it default to "notmaster" instead.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9952

Differential Revision: https://secure.phabricator.com/D14734