Buildbot could run builds on a single worker in order different to the order of commits

[gkistanova]

Here is the example of such case. The order of these two builds should be the other way around: https://lab.llvm.org/buildbot/#/builders/56/builds/1451 https://lab.llvm.org/buildbot/#/builders/56/builds/1452

Originally reported by @nikic - https://discourse.llvm.org/t/buildbot-started-commenting-prs-on-buid-failures/79738/13