Include first multiline commit in MR description for default template

What does this MR do and why?

I often spend a long time writing my commit messages, only to spend an extra step copying and pasting these into the merge request description. Having the text automatically inserted would save time.

This uses %{first_multiline_commit} to include the first multiline commit in the MR description.

Docs: https://docs.gitlab.com/ee/user/project/description_templates.html

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