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 the 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.
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