It's a bit annoying to only find those during a CI pipeline run, so
let's add a hook for it. Pre-commit doesn't seem to have something
useful for duplicate line detection so let's hack up a quick script
that can do this for us, together with the other whitespace checks in
the CI so we're consistent.
Excluded from the duplicate line checks are anything "Not C" and the
"include/" directory since we don't control those files.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1154>
