Do not run pipelines on obsolete changesets

We use Mercurial with the evolve extension enabled.

From time to time Pipelines runs a build on obsolete changes. One example is when merging a PR where the branch to merge in has obsolete heads. When doing the merge, Pipelines runs for the actual current head of the branch to merge (good) plus obsolete heads on that branch (bad).

Note that we use named branches for topic branches for PR.

I could provide an example Pipelines run in a private message.