Repositories API Compare call is missing WebURL

Calling Repositories.Compare returns a Compare object. Per the GitLab docs, the response includes a web_url field, however that is not in the Go type.

This should just need the field after here: https://github.com/xanzy/go-gitlab/blob/87336053bb9d01ec3fdb043114af22ac31667875/repositories.go#L211

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