[ci] Skip merge commits for length checks

[WinterSolstice8]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Downstreams have an issue where merge commits by us when we merge a PR can be too long, so we need to ignore merge commits during CI. This can trip false negatives where a PR contains a long merge commit, but we usually squash those out.

Steps to test these changes

I'm adding a positive test (too long commit) to this PR, and letting it spin and tell me I have a long commit. When it fails properly I'll attach proof that it failed, then remove that commit.

[WinterSolstice8]