make/scripts/update_copyright_year.sh is nifty, but of only limited use since it affects all change sets in a full year.

OTOH, contributors always need to manually adjust copyrights. That can be annoying. It should be possible to re-use update_copyright_year.sh, since it already does about 98% of the work.

Proposal:

- give the tool an option that limits the processed changesets to those in the current branch. Since most people will be following the branch-off-work-then-submit-PR flow.

So, if we could use something like `git log --no-merges 'master..HEAD'` as source for the to-be-processed changesets, that would be nice.

Optionally, we should also remove mercurial support.

If we add a "current branch only" mode, it would be good for that one to be the default, and for the "all changes that happened in year X" to be an one has to
state explicitly. That is because the latter mode will not be what most contributors want, and cancelling the script will leave garbage in /tmp

Maybe some saner argument processing, with named parameters, would be nice. Also a Usage text for wrong arguments or for "--help"