[FEATURE] Skip merge commits in pull requests

[GriceTurrble]

Merge commits can be identified by having two parent commits, so we add to the GraphQL query selecting PR commits to also return parents.totalCount. This shows any standard commit having 1 parent, and merge commits having 2.

We can then .filter() out those instances before mapping commit messages.

Should fix #75

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Note that TypeScript/JavaScript are not my strong suit. I would appreciate assistance in adding tests and perhaps bringing the same check to the push event.

[gilbertsoft]

Thanks for your PR @GriceTurrble . In the meantime I've extended the tests and also added the feature to the push event.

[simoneversienti]

Skipping merge commits is an interesting and useful feature. is this PR still valid?