Allow quoted strings as Scheme arguments to markup commands

input/regression/fret-diagrams-xo-label.ly

-\version "2.17.6"
+\version "2.21.0"
 
 \header {
   texidoc="
 The size, spacing, and symbols used to indicate open and muted strings
 can be changed.
 
 "
 }
 
 \layout { ragged-right = ##t }
 
 <<
   \chords {
     d1 |
     d1 |
     d1
   }
 
   \new Voice {
     \textLengthOn
 
      %% D major for guitar, terse style
     d'1 ^\markup {
-            \fret-diagram-terse #"x;x;o;2-1;3-2;2-3;"}
+            \fret-diagram-terse "x;x;o;2-1;3-2;2-3;"}
 
      %% D major for guitar, terse style
     \once \override TextScript.fret-diagram-details.top-fret-thickness = #5
     \once \override TextScript.fret-diagram-details.xo-font-magnification = #0.3
     d'1 ^\markup {
-            \fret-diagram-terse #"x;x;o;2-1;3-2;2-3;"}
+            \fret-diagram-terse "x;x;o;2-1;3-2;2-3;"}
 
      %% D major for guitar, terse style
     \override TextScript.fret-diagram-details.mute-string = #"M"
     \override TextScript.fret-diagram-details.open-string = #"*"
     \override TextScript.fret-diagram-details.xo-padding = #0.5
     d'1 ^\markup {
-            \fret-diagram-terse #"x;x;o;2-1;3-2;2-3;"}
+            \fret-diagram-terse "x;x;o;2-1;3-2;2-3;"}
 
  }
 >>