Add :git_graph_type to backmerge action

[mokagio]

What does it do?

What it says on the title. Follow up to @iangmaia 's suggestion from #594 (comment) because:

1. I want to add some tests to the logic
2. This is a low priority task so I won't be working on it with urgency, therefore I don't want to hold Use a Mermaid graph in the backmerge PR body #594 because of that

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations
• Add Unit Tests (aka specs/*_spec.rb) if applicable
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider.

[dangermattic]

	1 Warning
⚠️	Please add an entry in the CHANGELOG.md file to describe the changes made by this PR

	1 Message
📖	This PR is still a Draft: some checks will be skipped.

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