obsolete: compute unstable changeset An unstable changeset is a changeset *not* obsolete but with some obsolete ancestors. The current logic to decide if a changeset is unstable is naive and very inefficient. A better solution is to compute the set of unstable changeset with a simple revset and to cache the result. But this require cache invalidation logic. Simpler version goes first.

revset: add an `obsolete` symbol This predicate matches obsolete changesets. This is a naive implementation to be improved later.

push: refuse to push obsolete changesets This is a first version. Simple but not very efficient. Note that this changeset introduce the "obsolete" word in the UI.

push: accept revset argument for --rev

check-code: recognise %= as an operator

tests: do exclude what is expected