[native debian package] rake db:migrate is failing to find gitlab.yml when updating from 10.7.7 to 10.8.7

We install gitlab to /usr/share/gitlab (rails root) and symlink /usr/share/gitlab/config to /etc/gitlab and this was working fine till 10.7.7. Now when trying to update the package to 10.8.7, it fails when running rake db:migrate with this error.

rake aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - /etc/config/gitlab.yml
/usr/share/gitlab/config/initializers/1_settings.rb:4:in `<top (required)>'
/usr/share/gitlab/config/environment.rb:11:in `<top (required)>'
Tasks: TOP => db:migrate => environment
(See full trace by running task with --trace)

app_root is correctly set in /etc/default/gitlab and config/environment.rb should use this variable to load gitlab.yml correctly instead of using relative path.

cc @stanhu @balasankarc