Add index for merge_requests.merge_commit_shaadd-composite-index-on-merge-requests-merge-commit-sha

This is a composite index on (target_project_id, merge_commit_sha, id)
that allows queries such as the following to use a full backwards index
scan:

    SELECT "merge_requests".*
    FROM "merge_requests"
    WHERE "merge_requests"."deleted_at" IS NULL
    AND "merge_requests"."target_project_id" = 13083
    AND "merge_requests"."merge_commit_sha" = 'e80a893ff0ea8466099f6478183631af55933db2'
    ORDER BY "merge_requests"."id" DESC
    LIMIT 1;

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38507

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