compat: adapt to the new destspace argument of destmerge The argument is currently ignored in the topic case for simplicity, but we properly accept it and propagate it to the original function to prevent crash and behavior regression in the non-topic case.

stack: prevent crash when topic is rooted on nullid We exclude null from displayed base. This was causing a crash when trying to display its description (we'll still have to handle the empty description case, but it is now less urgent).

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.