Run a short respdiff test for all merge requests

Run a short respdiff test for all merge requests

Now that the respdiff tests can detect memory leaks, it is worth running
them for every merge request.  However, the existing respdiff-based
tests take a while to complete (about half an hour with our current CI
infrastructure), which does not make them a good fit for this purpose.
Add a new GitLab CI job, "respdiff-short", which uses a smaller query
set that gets processed within a couple of minutes on our current CI
infrastructure.  Rename the existing respdiff-based jobs to make
distinguishing them easier.