Reduce SQL timings of Projects::MergeRequestsController#show to a 99th percentile of less than 200 milliseconds

Currently the timings of this controller are as follows:

Requests per 12 hours	Percentage of Request Time spent in SQL	Mean	95th Percentile	99th Percentile
71 K	3.1%	160.61 ms	378.05 ms	446.94 ms

This controller's 99th percentile of SQL timings should be reduced to less than 200 milliseconds.

Blocked By

• https://gitlab.com/gitlab-org/gitlab-ce/issues/36301 (we need this for MRs as well if we want to really speed up MR pages)
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15200