Extend changelog API so the `to` argument is optional
This was suggested in #1546 (comment 507399508):
I'd suggest a future change to have the "to" be optional, and it default to HEAD if it is not passed in. That way it would then effectively ready as "create version X.Y.Z from branch
master
", taking all work from the last version to the latest commit.
In gitlab-org/release-tools!1364 (merged) we do this in Release Tools, but it's a sensible default to build into GitLab directly.