[Scripts/clean-check] Ignore git repos

Newer git versions update some cache in .git/index even on read-only
calls (like git status). So ignore any changes in .git when checking
make clean.

Scripts/clean-check

@@ -36,7 +36,7 @@ else
 fi
 
 ok=true
-diff "$diff_opts" -r "$clean_dir" "$top_dir" > "$diff" || ok=false
+diff "$diff_opts" -x .git -r "$clean_dir" "$top_dir" > "$diff" || ok=false
 
 if [ "$ok" == true ]; then
     echo "Source tree is unchanged. :]"