These changes come out of discussion in the team; we want to make use of
git trailers and improve the quality of our commit messages. Starting by
adding more hints to the commit suggestions. Includes some minor cleanup
of the hooks, and hopefully also improves tests so that they don't
interfere with interactions with non-Keyman repos.
Note: the scopes list may need maintenance as the list still includes
sub-scopes and 'oem'.
If no scope is found in the branch name, then don't include the
parentheses around the scope, as this breaks automatic merge messages
and the parentheses are unnecessary.
