Tell Git not to export the GitSetup directory

The `Utilities/GitSetup` directory is useful only in Git work trees
and so does not need to be distributed with source tarballs.

Utilities/.gitattributes

@@ -1,2 +1,3 @@
 /Git                    export-ignore
+/GitSetup               export-ignore
 SetupForDevelopment.sh  export-ignore