Fix return type for Reference.isHead (typed as boolean, but returns a number) by dsabanin · Pull Request #1498 · nodegit/nodegit

… number)

Both API docs and TypeScript typings say that Reference.isHead returns boolean, while in fact it's just a wrapper around Branch.isHead that returns 0 for false, 1 for true. 

This change makes the code do what published API already says it does. Branch.isHead is correctly typed in API as returning number.