Add a cleanup function to run before pipeline completion

[thepushkarp]

During each pipeline run, various parts of the pipeline can change the state of various objects, such as the HEAD pointer of the repositories that the pipeline uses.

Right before the completion of each run, there should be a "cleanup" function that would undo all of these changes of that run so that these repositories and other dependent objects would be in the desired working state when the next pipeline run is invoked.

Related: #49