Extract #uncached_files logic

lib/gitlab/diff/highlight_cache.rb

@@ -31,10 +31,6 @@ module Gitlab
       #   IO generated by N+1's (1 writing for each highlighted line or file).
       #
       def write_if_empty
-        diff_files = @diff_collection.diff_files
-        cached_diff_files = read_cache
-        uncached_files = diff_files.select { |file| cached_diff_files[file.file_path].nil? }
-
         return if uncached_files.empty?
 
         new_cache_content = {}
@@ -59,6 +55,13 @@ module Gitlab
 
       private
 
+      def uncached_files
+        diff_files = @diff_collection.diff_files
+        cached_diff_files = read_cache
+
+        diff_files.select { |file| cached_diff_files[file.file_path].nil? }
+      end
+
       # Given a hash of:
       #   { "file/to/cache" =>
       #   [ { line_code: "a5cc2925ca8258af241be7e5b0381edf30266302_19_19",