[dmd-internals] Add auto-merge label which is set by the auto-tester?

[Andrej Mitrovic via dmd-internals]

In case there's a lot of pulls being tested and already a set of them
were enabled with auto-merge, I'd like to filter these PRs out of the
PR list on github. Labels seem like a good tool for this. Perhaps the
auto-tester could label a PR with a "auto-merge" label so we can
filter that PR when we're on the hunt for reviewing PRs which haven't
been approved for merging yet.