The shortcut I'm presenting as a solution is slightly long but will work for sure.
As soon as you open the file from the source control panel use the following shortcuts to copy the file's relative path
Windows/Linux ctrl+k Mac cmd+k immediately followed by ctrl+shift+c \ cmd+shift+c
This will copy the relative path of the file you can then use the command ctrl+f4 / cmd+f4 to close the opened history view of the file
then open the command palette using ctrl+shift+p / cmd+shift+p and remove the > character and paste the relative path of the file using ctrl+v / cmd+v
When you open the file in the diff view take care not to click the cursor within the editor as this will move to editor focus. When in editor focus the command to copy the relative path will not work. However this can be fixed by changing this in the keyboard shortcut settings.