Changing the default branch on an imported repository without "master" fails

Issue Summary

When trying to update the default branch after importing a repository, it will fail if the imported repository does not have a "master" branch (the default "default" branch for a newly created repository).

Steps to Reproduce

1. Import a repository
2. See the "refs/head/master does not exist" message on the Source view
3. Navigate to repository settings
4. Attempt to update the default branch

Expected Results

The default branch is updated.

Actual Results

Receive a "refs/heads/master is set as the default branch, but this branch does not exist"

Workaround

1. Create a "master" branch
2. Update the default branch
3. Delete the "master" branch