Allow composition of pure-ASCII strings in the mode line

* src/composite.c (Fcomposition_get_gstring): Allow unibyte
strings if they are pure ASCII, by copying text into a
multibyte string.

src/composite.c

@@ -1746,7 +1746,18 @@ should be ignored.  */)
       CHECK_STRING (string);
       validate_subarray (string, from, to, SCHARS (string), &frompos, &topos);
       if (! STRING_MULTIBYTE (string))
-	error ("Attempt to shape unibyte text");
+	{
+	  ptrdiff_t i;
+
+	  for (i = SBYTES (string) - 1; i >= 0; i--)
+	    if (!ASCII_CHAR_P (SREF (string, i)))
+	      error ("Attempt to shape unibyte text");
+	  /* STRING is a pure-ASCII string, so we can convert it (or,
+	     rather, its copy) to multibyte and use that thereafter.  */
+	  Lisp_Object string_copy = Fconcat (1, &string);
+	  STRING_SET_MULTIBYTE (string_copy);
+	  string = string_copy;
+	}
       frombyte = string_char_to_byte (string, frompos);
     }