R1:f60548d0818c

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…