Additional stable branch
We've had the `heptapod` and `heptapod-stable` branches for a while, now.
With Heptapod releases now being closer to upstream GitLab releases, the stable branch stays alive for more time. A consequence is that we have more overlap: today, Heptapod 0.19 is a week before EOL, 0.20 has 2 months yet we are already working on the future 0.21. A week is not that much, but if all goes well, it should be a month next time.
So, we need one more branch. Let's call it `heptapod-oldstable`, starting with the current 0.19.
## Task list:
- [x] ci-images/heptapod-base
- [x] ci-images/heptapod-gitaly
- [x] ci-images/mercurial (no need for the `hg-default` and `hg-stable` variants
- [x] Rails app
- [x] Heptapod Shell
- [x] py-heptapod
- [x] HGitaly
- [x] HDK
- [x] Functional tests
- [x] Omnibus
- [x] Workhorse (just for uniformity, `heptapod-stable` will be closed as soon as Heptapod 0.20 becomes the new stable.
issue