Add %{first_multiline_commit} variable squash and merge templates

Problem to Solve

With the addition of #345275 (closed) and previously !64437 (merged) that introduced support for templates for squash and merge, not all behavior of previous squash configuration is supported. Namely, the ability to use the first_multiline_commit as the commit message.

Proposal

With #345275 (closed) we now have a way to customize the default squash commit message per project.

To support the old default of using the “first multi-line commit” as the squash commit message, we should support a new variable %{first_multiline_commit} in the squash commit message template.