Use class methods to create native object shims

CoffeeScript 1.5 complains if a constructor has
a returns a value.

src/app/git.coffee

@@ -24,7 +24,7 @@ class Git
     ignore: 1 << 14
 
   constructor: (path, options={}) ->
-    @repo = new GitRepository(path)
+    @repo = GitRepository.open(path)
     refreshIndexOnFocus = options.refreshIndexOnFocus ? true
     if refreshIndexOnFocus
       $ = require 'jquery'