Use `default_merge_message/REBASE_TEMPLATE.md` for amending the message of the
last commit in the list of commits that was merged. Previously this template
was mentioned in the documentation but not actually used.

In this template additional variables `CommitTitle` and `CommitBody` are
available, for the title and body of every commit.

Ideally the message of every commit would be updated using the template, but
doing an interactive rebase or merging commits one by one is complicated, so
that is left as a future improvement.