changes display wrong

Steps to reproduce

• create a new group (eg testGroup)
• create a new project under the group, and initialize it with some file (eg testProject)
• fork the new project to your own namespace (eg root/testProject)
• make some changes to the project in your namespace
• create a "NEW MERGE REQUEST" to the project in newly created group ( root/testProject ---->>>> testGroup/testProject)
• click the button "Compare branches and continue"
• the error appears the changes number is 0 which should be some other number that matches the changes

Some other descriptions

I have tested some versions of Community Edition until the lastes version ( gitlab-ce-13.4.3-ce.0.el7.x86_64.rpm) , the bug still exists .
So , it will be apprecitated to fix it to make the gitlab more friendly .
Thanks