add method for commit references

[felsgaertner]

The API provides (since 10.6) a way to filter the type of references for a commit.

v4-0.17 complains if this filter is added to the method call commit(). I would be nice to have this in the module.

[bluefeet]

The commit() method is generated for this endpoint:

- commit: commit = GET projects/:project_id/repository/commits/:commit_sha

Which is documented here:

https://docs.gitlab.com/ce/api/commits.html#get-a-single-commit

The endpoint you linked to is an entirely new enpoint that this client does not yet support. I'll update the ticket title to reflect this.