Add spec for CompareService
Add spec for CompareService

The return result for the source or target branch being missing was
ambiguous before. It should be an empty compare to indicate no
comparison could be done.

iiet/iiet-git,stoplightio/gitlabhq,iiet/iiet-git,iiet/iiet-git,mmkassem/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq