Hide .designer.cs files

VS creates a bunch of files that, while important to version, are often also
huge and boring. We should suppress them.

lib/linguist/generated.rb

@@ -55,8 +55,9 @@ module Linguist
         minified_javascript? ||
         compiled_coffeescript? ||
         xcode_project_file? ||
-        generated_net_docfile? ||
         generated_parser? ||
+        generated_net_docfile? ||
+        generated_net_designer_file? ||
         generated_protocol_buffer?
     end
 
@@ -143,6 +144,16 @@ module Linguist
         lines[-2].include?("</doc>")
     end
 
+    # Internal: Is this a codegen file for a .NET project?
+    #
+    # Visual Studio often uses code generation to generate partial classes, and
+    # these files can be quite unwieldy. Let's hide them.
+    #
+    # Returns true or false
+    def generated_net_designer_file?
+      name.downcase =~ /\.designer\.cs$/
+    end
+
     # Internal: Is the blob of JS a parser generated by PEG.js?
     #
     # PEG.js-generated parsers are not meant to be consumed by humans.