Add failing tests for issue #30

Zero-content pull/fetch after push triggers a notes commit on
fast-import, which git applies successfully, but complains on stderr.

error: refs/notes/hg-84b3865b750a567acb16929c21e14c4a45a5639b does not point to a valid object

New tests:

* test_simple_push_updates_notes
  checks for automatic update after push followed by zero-content
  pull/fetch

* test_simple_push_updates_notes_after_contentful_pull
  checks for eventual update after a fetch/pull that provides content on
  a non-notes ref
Commit notes only when there is at least one non-notes commit (fixes #30)

Git is rejecting commits to refs/notes/hg-* when there are no other
commits in the fast-imports stream. I can't tell if this is intended
behavior (why would it be?) but deferring application of the notes works
around the problem.

Fixes test_simple_push_updates_notes_after_contentful_pull.

Leaving xfail test_simple_push_updates_notes to track when we can apply
notes more eagerly without messy stderr.
Merge pull request #32 from jedbrown/notes-in-empty-fetch

Notes in empty fetch (issue #30)