Many verbose encoding mismatch warnings

Currently warnings are thrown by concatenate-source-op every time ASDF is loaded if UTF-8 is the default encoding because the ASDF source is ASCII which is not equal to UTF-8. The warnings have the following form (newlines added for readability):

   #<ASDF/CONCATENATE-SOURCE:MONOLITHIC-CONCATENATE-SOURCE-OP > uses encoding UTF-8 but has sources that use these encodings:
(NIL (driver asdf/driver)) (NIL (backward-driver asdf/driver))...

They quickly flood the REPL or STDOUT harming readability of important warnings.

The following adds a special case to the encoding equality check so that ASCII is recognized as a subset of UTF-8.

modified   concatenate-source.lisp
@@ -69,7 +69,8 @@ into a single file"))
           :append
           (when (typep c 'cl-source-file)
             (let ((e (component-encoding c)))
-              (unless (equal e encoding)
+              (unless (or (equal e encoding)
+                          (and (equal e :ASCII) (equal encoding :UTF-8)))
                 (let ((a (assoc e other-encodings)))
                   (if a (push (component-find-path c) (cdr a))
                       (push (list e (component-find-path c)) other-encodings)))))

Thanks, Eric