Using the same approach as we have for code style checks (the
`style-check-diff` CI job), check the diff for any banned keywords like
‘TODO’, and also check the commit messages.
The keyword ‘TODO’ is often used by developers to indicate a part of a
commit which needs further work, and hence which shouldn’t yet be merged.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Fixes: #1551
