Commit class does not expose the branches-where-head endpoint

It is sometimes required to identify any branches of which a given commit ID is a member of.

GitHub API provides such a method as documented here: https://docs.github.com/en/rest/commits/commits?apiVersion=2022-11-28#list-branches-for-head-commit

Propose adding a new member function `branchesWhereHead()` to the `Commit` object to expose this

Start at the Commit object and compare its existing API methods with the linked GitHub “List branches for head commit” endpoint. Add the branchesWhereHead() entry point and verify that it exposes that endpoint with the expected response shape.

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