Controller Projects::MergeRequestsController#merge executes more than 100 SQL queries

Description

This controller executes more than 100 SQL queries, the maximum number of queries allowed as per MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16466.
The number of queries should be reduced to less than 100 and the corresponding whitelisted sections of code (Gitlab::QueryLimiting.whitelist('...')) should be removed.

Impact

This query was reduced by 50% with !57346 (merged), see details in #4792 (comment 535160102).