Select default branches more effectively in Diffusion

Summary: If you have an empty value saved in the "default branch" field, we default to empty string (or null, or whatever) instead of the correct default.

Test Plan:

• Looked at a Git repo with an empty default branch, got a default to "master" instead of an error.
• Looked at a Mercurial repo with an empty default branch, got a default to "default" instead of an error.

Reviewers: btrahan, csilvers

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1237

Differential Revision: https://secure.phabricator.com/D2512