Fix N+1 for searching commits

What does this MR do?

When searching commits, for every unique commit author we execute the following query (N+1)

SELECT "users".* FROM ((SELECT "users".* FROM "users" WHERE (LOWER("users"."email") IN (LOWER('nicolo.mezzopera@gmail.com'))) AND "users"."confirmed_at" IS NOT NULL) UNION (SELECT "users".* FROM "users" INNER JOIN "emails" ON "emails"."user_id" = "users"."id" WHERE "emails"."email" = 'nicolo.mezzopera@gmail.com' AND "users"."confirmed_at" IS NOT NULL AND "emails"."confirmed_at" IS NOT NULL)) users

This MR fixes that and preloads all authors in 1 query in the prepare_commits_for_rendering method, which is called by SearchServicePresenter

Relates to gitlab-org/search-team/team-tasks#36 (closed)

#327006 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #327006 (closed)