git kill branchname

@charlierudolph Aaaaahhhh, for some reason some commits meant for this PR ended up in an already merged PR. I was messing around with `git extract` while being distracted on the resource planning call, so that's my (super lame) excuse... Anyways, please take a look at https://github.com/Originate/git-town/commit/738e3e76158a4803c5695abf71349b21d02033f5, and comment there. I will implement the corresponding fixes in this pull request, and once this gets merged its all going to be kind of okay again...  :smile: 
This specifies the case when my current changes conflict with existing changes on the main branch. 

`git kill` checks out the main branch, which will break in this case. This line motivates stashing open changes during killing, as implemented in https://github.com/Originate/git-town/blob/kg-git-kill-branchname/git-kill#L51. Without this line, the stash is unnecessary according to the specs.