Cleanup some pending changes.

* tests/test_upload.py:
Fix imports.
(TestUploadFromRemoteBranch.setUp): Refactor some setup common to
all tests.

* __init__.py:
(CannotUploadToWorkingTree): Renamed from
CannotUploadToWorkingTreeError, no need to prefix with error, we
know that.
(DivergedUploadedTree): Replace DIvergedError.
(cmd_upload.run): Simplify locking.