The file watcher knows which files exist, so don't use slow File.exists?

Possible solution for #903

middleman-core/lib/middleman-core/core_extensions/file_watcher.rb

@@ -56,6 +56,7 @@ module Middleman
       class API
 
         attr_reader :app
+        attr_reader :known_paths
         delegate :logger, :to => :app
 
         # Initialize api and internal path cache
@@ -142,6 +143,12 @@ module Middleman
           reload_path(path, true)
         end
 
+        def exists?(path)
+          p = Pathname(path)
+          p = p.relative_path_from(Pathname(@app.root)) if !p.relative?
+          @known_paths.include?(p)
+        end
+
       protected
         # Whether this path is ignored
         # @param [Pathname] path