Previously `test_fetch_emits_canonical_ref_update` was passing by
catching a leftover `CanonicalRefUpdated` event from the initial
repository fetch. Furthermore, the new commit was written to the
un-namespaced root. It can be proven to be the case by commenting out
the `alice.commit_to(...)` line and watching the test still pass.
Fix the test by committing to the namespaced default branch which will
trigger the `CanonicalRefUpdated` that is expected by Bob. Drains events
so we don't catch the wrong event, and explicitly announces.
80d30d3b59