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.