fix undefined old_head exception
fix undefined old_head exception

If the try block throws, the later context doesn't have the referenced
old_head. Give it an empty value to avoid the error.

This fixes the current failing test_project_update
(tests.test_import.UnprivilegedImportTest).

patchew-project/patchew,patchew-project/patchew,patchew-project/patchew,patchew-project/patchew