I cannot get the "compare" URL to work. Here is a hack to make it possible to compare a file against a different version.
Go to the file view on one of the commits. On the file's menu bar, next to the "Download" icon, there is the "Open raw" button. Click on it and opens the raw file. Select All and Copy.
Go to the file view of the other commit you want to compare. On the file's menu bar, click on the "Edit" button and select "Edit single file" (you may need to fork the project or create a private branch if it is locked).
Paste over the raw file and click on "Preview changes"
It should show you the changes, which is the diff between the 2 versions.