Fix an issue where Diffusion could fatal if the default branch was deleted

Summary: See PHI234. In T12931 we improved the behavior of Diffusion when a repository's default branch is set to a branch that does not exist, but in T11823 the way refcursors work changed, and we can now get a cursor (just with no positions) back for a deleted branch. When we did, we didn't handle things gracefully.

Test Plan:

• Set default branch to a deleted branch, saw nice error instead of fatal.
• Set default branch to a nonexistent branch which never existed, saw nice error.
• Set default branch to existing "master", saw repository normally.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18811