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