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

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

@@ -59,9 +59,9 @@ module Middleman::CoreExtensions
         @cache[p] ||= begin
           in_file = frontmatter_and_content(p)
 
-          external_file = frontmatter_and_content("#{p}.frontmatter")
+          return in_file unless @app.files.exists?("#{path}.frontmatter")
 
-          return in_file if external_file.nil?
+          external_file = frontmatter_and_content("#{p}.frontmatter")
 
           [
             external_file[0].deep_merge(in_file[0]),
@@ -145,7 +145,7 @@ module Middleman::CoreExtensions
           path
         end
 
-        return nil unless File.exists?(full_path)
+        return nil unless @app.files.exists?(full_path)
 
         data = {}
         content = nil

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

@@ -476,7 +476,7 @@ module Middleman
             end
 
             # If we found one, return it and the found engine
-            if found_path || (File.exists?(on_disk_path) && !File.directory?(on_disk_path))
+            if found_path || files.exists?(on_disk_path)
               engine = found_path ? File.extname(found_path)[1..-1].to_sym : nil
               [ found_path || on_disk_path, engine ]
             else