User stories for git-hack

@charlierudolph @allewun adding user stories that describe end-user value
This is a "primary" feature spec. It needs to motivate the whole `git hack` feature, and justify its existence. Simply not interrupting the user flow is not enough here. If the feature didn't exist at all, my flow would also not be interrupted. The point of why we have git hack is that my work can be distributed onto feature branches easily. We do that to make code reviews more efficient, which ultimately leads to better team productivity. 

Once we re-arrange specs so that only "primary" specs are left in the directory this will become more clear.