- Simplify if-else expression to `NotFoundOrServerError`.
- I cannot find an existing scenario where `Getdiff` returns an error
and where it therefore should show a 404 error in the context of
rendering a diff of a commit. So simply return it as an Internal Server
Error, this also helps with debugging if an actual error occurs
here (404 errors are only logged at the DEBUG level).
- The first change is already covered under existing testing, the second
change is not trivial to test.
4c123d8e3b