Only check MR pipelines

When marge checks an MR's pipeline status, it should only check for pipelines with merge_request_event source and ignore pipelines with other sources, especially those manually triggered to only test certain jobs. See ghc#25404 (closed).

requested review from @bgamari, @mpickering, and @chreekat

Makes sense to me, though the only way I know to test it is to deploy the changes and see what happens. I could give it a shot next week, unless someone else wants to try it.

Sounds good! Looking forward to deployment.

Doing it now!

So far so good -- she picked up the same two MRs she was previously merging.

mentioned in commit 013d590e

merged

mentioned in issue ghc#25404 (closed)