repo_set: avoid triggering redundant builds

fetch_mirrors uses repo_set to monitor branches and trigger builds.

For active branches (eg mesa main), most merge requests will trigger a build when a build is still running. We want to limit the queue length to 1, because any equeued build will test the most recent commit when it starts.

Without this governor, many enqueued builds will drain by re-testing the last commit before a quiet period. Mesa CI re-uses existing results, so this is quick to 'execute' on the test systems. However, the re-used results will send duplicate/redundant test output to mesa-ci.01.org. That result database struggles to scale to the throughput of the CI for active projects.