MR wrongly claims that "source branch does not exist"

Summary

MR claims that "source branch does not exist" but displays the diff and commits just fine

Steps to reproduce

1. Fork a project
2. push a new branch onto the fork
3. create a MR merging the new branch into master

Example Project

Not able to reproduce on GitLab.com yet

What is the current bug behavior?

The "Merge" button is gray, and says "source branch does not exist. Please restore it or use a different source branch"

What is the expected correct behavior?

The "Merge" button should be clickable and working.

Relevant logs and/or screenshots

Results of GitLab environment info

Expand for output related to GitLab environment info

bbleae@mtcode:~$ sudo gitlab-rake gitlab:env:info
System information
System:         Ubuntu 14.04
Current User:   git
Using RVM:      no
Ruby Version:   2.4.5p335
Gem Version:    2.7.6
Bundler Version:1.16.2
Rake Version:   12.3.1
Redis Version:  3.2.12
Git Version:    2.18.1
Sidekiq Version:5.2.1
Go Version:     unknown
GitLab information
Version:        11.4.5
Revision:       f5536c6
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://git.mtintl.net
HTTP Clone URL: https://git.mtintl.net/some-group/some-project.git
SSH Clone URL:  git@git.mtintl.net:some-group/some-project.git
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:
GitLab Shell
Version:        8.3.3
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Results available below.

Expand for output related to the GitLab application check

bbleae@mtcode:~$ sudo gitlab-rake gitlab:check SANITIZE=true
Checking GitLab Shell ...
GitLab Shell version >= 8.3.3 ? ... OK (8.3.3)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:root, or git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
3/7 ... ok
3/8 ... ok
3/9 ... ok
3/10 ... ok
3/12 ... ok
3/13 ... ok
5/14 ... ok
5/15 ... ok
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 Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Reply by email is disabled in config/gitlab.yml
Checking LDAP ...
LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab ...
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? ... yes
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: ...
3/7 ... yes
3/8 ... yes
3/9 ... yes
3/10 ... yes
3/12 ... yes
3/13 ... yes
5/14 ... yes
5/15 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.4.5)
Git version >= 2.9.5 ? ... yes (2.18.1)
Git user has default SSH configuration? ... yes
Active users: ... 7
Checking GitLab ... Finished