Viewing already merged Merge Requests fails with 500 after running BFG repo cleaner

Summary

Viewing any of the existing merge requests in our project causes the page to return a 500 error.

Steps to reproduce

This started happening after I rewrote some repo history on master using BFG Repo Cleaner. I roughly followed instructions at https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html.

Example Project

This is happening on the flow-server project.

What is the current bug behavior?

Viewing a merge request gives a 500 error screen

What is the expected correct behavior?

Should see the MR.

Relevant logs and/or screenshots

https://www.dropbox.com/s/evruca6x51vhi3r/Screenshot%202019-08-15%2012.30.46.png?dl=0

Output of checks

This bug happens on gitlab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:env:info)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)