Dogfood squash commit template for gitlab-org/gitlab

Feature to Dogfood

GitLab has a new feature #345275 (closed) that allows setting a default squash commit message using variables.

Goals

Dogfood this feature by defining a default squash commit message template for gitlab-org/gitlab.

Expected benefits

Dogfooding this feature should help gitlab-org/gitlab to preserve its Changelog git trailers which currently, by default, are not included in the squash commit message. See the variable %{first_multiline_commit}.

It might also aid in improved discoverability of the associated issues and merge request via the commit message, using the %{issues} and %{reference} variables.