test: add topic on the first 2 changesets too This will be used to add a test for topic rooted on nullid. We introduce this in its own changeset to reduce the noise if the actual changeset.

test: remove comment about new head warning The new head warning was buggy. This was fixed in bdc5bb223b50.

stack: also change the indexing of the t# reference Changeset 61f36480740f changed the index displayed (we now index from 1), but forgot to update the logic used to resolve the 't#' name. This changeset fixes it.