Differentiate polling from find_new_files to remove double reads in build mode

middleman-core/lib/middleman-core/sources/source_watcher.rb

@@ -1,6 +1,7 @@
 # Watcher Library
 require 'listen'
 require 'middleman-core/contracts'
+require 'digest'
 
 # Monkey patch Listen silencer so `only` works on directories too
 module Listen
@@ -177,6 +178,16 @@ module Middleman
       @listener = nil
     end
 
+    Contract ArrayOf[Pathname]
+    def find_new_files!
+      new_files = ::Middleman::Util.all_files_under(@directory.to_s)
+          .reject { |p| @files.key?(p) }
+
+      update(new_files, [])
+
+      new_files
+    end
+
     # Manually trigger update events.
     #
     # @return [void]