Exclude merge commit from multi-line commit selection in Danger

What does this MR do?

When a MR is set to be squashed, we shouldn't pick a merge commit to be used as the squash commit.

With this change, a merge commit will only trigger a warning, not a failure.

This should improve cases like !21453 (comment 283792959).

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done