repository.fetch(same_url_repository, revision_id=revision_added_after_write_lock) fails

While writing import-upstream for bzr builddeb I found a bug in repository.fetch.

If you do this:
init-repo .
init b
init c
then in python
b.lock_write()
b.branch.repository.all_revision_ids()
c.lock_write()
revid = c.commit('')
b.branch.repository.fetch(c.repository, revision_id=revid)

NoSuchRevision will be raised, because b doesn't have the revid in its indices, and hasn't refreshed its data. Calling refresh_data() would fix it, but it can't be called in a write group. I've filed bug 574223 on bzr-builddeb about having ugly code as a result of this.