Run a short respdiff test for all merge requests

Run a short respdiff test for all merge requests

Running a respdiff test for every merge request would be useful for
catching protocol-breaking changes before they are applied to the source
code.  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.

(cherry picked from commit 31ee43a314f17b433909a049fafa01200bac14ca)