MRs should be properly linked to deployments when fast-forward merge method is selected.

Actual: When Fast-forward merge method is selected MRs are not linked to deployments.

Expected: MRs should be properly linked even with FF merge method.

All features which rely on deployments-MRs link don't work. For example "Lead Time for Changes" DORA metric.

LinkMergeRequestsService detects MRs by their merge_commit_sha only which is absent when merge method is fast-forward. We should extend detection logic to include other related commits. There is a handy scope for that.

Please pay attention to performance impact of queries.