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Skip mergeRequestMergeStatusUpdated trigger when not needed

Patrick Bajao requested to merge 396255-skip-merge-status-trigger-refresh-service into master

What does this MR do and why?

In MergeRequests::RefreshService, we mark all affected MRs as unchecked. In MergeRequest#merge_status state machine, we have an after_transition block that triggers a GraphQL subscription when MR is marked as unchecked.

There's a scenario wherein a single push can result to a lot of SQL queries in a single trigger. For example, if someone pushes to main and there are 1K MRs targetting that branch, all those MRs will be marked as unchecked. That can result to a 1K SQL queries when GraphQL subscription is triggered.

To prevent that, a skip_merge_status_trigger flag is added to MergeRequest and it is set conditionally in MergeRequests::RefreshService. We only need to trigger the subscription when the push is for the MR's source branch and project. This way we can still update the merge widget in real-time when a push occurs to the MR's branch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #396255 (closed)

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