[performance] Eliminate calls to PyGithub __completeIfNeeded

[shevron]

During the analysis for #17, we detected many cases where PyGitHub is missing some value that it needs, thus calling __completeIfNeeded which triggers an additional API call.

We should investigate all cases where this happens, to see if it can be avoided by proving some additional values when instantiating objects or calling methods, thus avoiding PyGitHub's need to fetch those values from the server.