GitHub "merged by" worker updates merge request `updated_at` field

From !48632 (comment 456566963), it appears the newly-introduced "merged by" worker for GitHub updates the updated_at timestamp for the merge request, so old, merged merge requests look like this:

$ grep -B 5 -A 5 updated_at /tmp/import.log  | grep 4215
  MergeRequestDiff Create (2.0ms)  INSERT INTO "merge_request_diffs" ("state", "merge_request_id", "created_at", "updated_at", "external_diff") VALUES ('empty', 4215, '2020-11-29 05:56:45.140171', '2020-11-29 05:56:45.140171', 'diff-') RETURNING "id"
  Note Create (2.3ms)  INSERT INTO "notes" ("note", "noteable_type", "author_id", "created_at", "updated_at", "project_id", "noteable_id", "discussion_id", "note_html", "cached_markdown_version", "system") VALUES ('*Merged by: kawamuray*', 'MergeRequest', 1, '2017-07-05 18:11:32', '2020-11-29 05:56:53.111769', 225, 4215, 'ceb00c796c845eb80831a1164d20785ac0b5ec1a', '<p data-sourcepos="1:1-1:22" dir="auto"><em>Merged by: kawamuray</em></p>', 1703937, FALSE) RETURNING "id"
  MergeRequest Update (2.8ms)  UPDATE "merge_requests" SET "updated_at" = '2020-11-29 05:56:53.172860' WHERE "merge_requests"."id" = 4215
  MergeRequest::Metrics Load (3.3ms)  SELECT "merge_request_metrics".* FROM "merge_request_metrics" WHERE "merge_request_metrics"."merge_request_id" = 4215 AND "merge_request_metrics"."target_project_id" = 225 LIMIT 1
  MergeRequest::Metrics Create (0.5ms)  INSERT INTO "merge_request_metrics" ("merge_request_id", "created_at", "updated_at", "target_project_id") VALUES (4215, '2020-11-29 05:56:53.281972', '2020-11-29 05:56:53.281972', 225) RETURNING "id"

We might be able to solve this by setting importing = true as described in !48632 (comment 456602968).

/cc: @kassio