No prompt with Haskell and git 2.17 and initial repo

[starcraftman]

Spoke too soon on issue #17 . Seems Haskell parsing for branch from porcelain also broke post 2.17 git. No prompt appears at all.

On git < 2.17, porcelain branch line on initial repo is: ## Initial commit on master
On git >= 2.17, porcelain branch line on initial repo is: ## No commits yet on master

Haskell missing latter support. Simple fix.

To be clear, the test case is a firstly initialized repo (i.e. all you did was git init). Probably Haskell test suite missing the case since Travis now using 2.17.

[starcraftman]

I think this is fixed in #19, close it on merge and verify.