prev: allow to update to 't0' from 't1' When on the first changeset of a topic, one can move back one changeset to end up on t0. The new working copy parent to not belong to the topic, but the topic is still active. The next commit will create a new root to the active topic. You can get the same result using `hg up t0`

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t