This is part of the transition away from SVN to Git.
Thanks for changing this, Petr!
Looks good to me with one unclear place around handling the clean property. Please see my comment inline. If that behavior is intentional, it might worth a comment next to it explaining the reasoning.
With this addressed feel free to commit, please.
Are you sure you always want to do a clean build, unless otherwise requested from the bot Web UI?
Yes this is intentional. We're building a full toolchain and we've seen issues in the past when not doing a full build. I might come up with a better solution, and then re-evaluate this. Our builders are powerful enough that this shouldn't be an issue hopefully.