Add Project.proto.relativize method to make an absolute path relative to the project root

src/app/project.coffee

@@ -22,3 +22,5 @@ class Project
     filePath = fs.join(@path, filePath) unless filePath[0] == '/'
     fs.absolute filePath
 
+  relativize: (path) ->
+    path.replace(@path, '')

src/app/root-view.coffee

@@ -115,7 +115,7 @@ class RootView extends View
       @fileFinder = null
     else
       @project.getFilePaths().done (paths) =>
-        relativePaths = (path.replace(@project.path, "") for path in paths)
+        relativePaths = (@project.relativize(path) for path in paths)
         @fileFinder = new FileFinder
           paths: relativePaths
           selected: (relativePath) => @open(relativePath)