A few MRs present consistent `find_commit` N+1 for MR controller `show` and `show.json` endpoints

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18991 fixed a big part of existing find_commit N+1's for MR discussion page.

But there are a few MRs such as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5554 that consistently requests Gitaly's find_commit endpoint in both Projects::MergeRequestsController#show and Projects::MergeRequestsController#show.json in a N+1 manner.

Guesses:

• Projects::MergeRequestsController#show could be due to banzai commit parser

cc @smcgivern