Merge request view not loading changes tab when users bump a new revision of a submodule

Summary

Merge request view not loading changes tab when users bump a new revision of a submodule.

Steps to reproduce

Create a merge request contains a commit which implements a submodule bump to the main repository.

Example Project

Bug exist for 12.1.0-ee.0 version, we have a self-hosted instance having this issue and I managed to reproduce it also in my localhost setup.

Sharing a merge request created in community edition as reference for reproducing the problem onurcelep/test-main-repository!1

What is the current bug behavior?

Changes tab on the MR view can not be loaded.

What is the expected correct behavior?

MR changes should be listed for each commit in the MR

Relevant logs and/or screenshots

image

Output of checks

This bug happens on self-hosted GitLab instance

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.6.3p62
Gem Version:	2.7.9
Bundler Version:1.17.3
Rake Version:	12.3.2
Redis Version:	3.2.12
Git Version:	2.21.0
Sidekiq Version:5.2.7
Go Version:	unknown


GitLab information
Version:	12.1.0-ee
Revision:	1f2e6f3f6d8
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	9.6.11
URL:		http://localhost:8082/gitlab
HTTP Clone URL:	http://localhost:8082/gitlab/some-group/some-project.git
SSH Clone URL:	git@localhost:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:


GitLab Shell
Version:	9.3.0
Repository storage paths:
  • default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check 
Checking GitLab subtasks ...


Checking GitLab Shell ...


GitLab Shell: ... GitLab Shell version >= 9.3.0 ? ... OK (9.3.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK


Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful


Checking GitLab Shell ... Finished


Checking Gitaly ...


Gitaly: ... default ... OK


Checking Gitaly ... Finished


Checking Sidekiq ...


Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1


Checking Sidekiq ... Finished


Checking Incoming Email ...


Incoming Email: ... Reply by email is disabled in config/gitlab.yml


Checking Incoming Email ... Finished


Checking LDAP ...


LDAP: ... LDAP is disabled in config/gitlab.yml


Checking LDAP ... Finished


Checking GitLab App ...


Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... skipped (no tmp uploads folder yet)
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
2/1 ... yes
1/2 ... yes
2/3 ... yes
2/5 ... yes
2/6 ... yes
2/7 ... yes
1/8 ... yes
2/9 ... yes
2/10 ... yes
1/11 ... yes
2/12 ... yes
2/13 ... yes
2/14 ... yes
2/15 ... yes
2/16 ... yes
2/17 ... yes
1/18 ... yes
5/19 ... yes
2/20 ... yes
1/21 ... yes
2/22 ... yes
1/23 ... yes
1/24 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.3)
Git version >= 2.21.0 ? ... yes (2.21.0)
Git user has default SSH configuration? ... yes
Active users: ... 3
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished

Possible fixes

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