Improve translation check action

Few things came up after seeing the script in action. Short intro how it works:

1. Checkout the branch of the fork (newest commit) to the pr directory
2. Checkout the master branch of the fork (not OpenRCT/Localisation)
3. Calculate and checkout the merge-base of the two as master

Using the image as example, pr will point to ca1ff5e and master will point to 0339427

Problems with that:

• If the pull request is created using the fork master branch it will not detect any changes (as the merge-base is the same as the newest commit). Example here: #1996. Already mentioned here: https://github.com/OpenRCT2/Localisation/pull/1992#issuecomment-711331634
• If the fork master branch is not rebased it will not contain the newest or not contain at all the check script. Example here: #1997. This should be easy to fix: additionally checkout the master of OpenRCT2/Localisation and run the script from there
• Do not post a comment if there are no changes

Start by tracing the translation check action and its checkout sequence for the PR branch, fork master, and merge base. Use examples #1996 and #1997 to verify the failure cases, then check that the script runs from OpenRCT2/Localisation master and that no comment is posted when there are no changes.