Allow music functions to recognize strings as markup.

input/regression/music-function-string-markup.ly

Index: input/regression/music-function-string-markup.ly
diff --git a/input/regression/music-function-string-markup.ly b/input/regression/music-function-string-markup.ly
new file mode 100644
index 0000000000000000000000000000000000000000..e3a80358be6bb9b8bf39e1716d0f62f6d8c303e4
--- /dev/null
+++ b/input/regression/music-function-string-markup.ly
@@ -0,0 +1,17 @@
+\version "2.13.8"
+
+\header {
+  texidoc = "Music functions accept strings as markup arguments
+when using the type predicate @code{markup?}
+"
+}
+
+testFunc =
+#(define-music-function (parser location text music) (markup? ly:music?)
+   ;; dummy function, does nothing
+   music)
+
+\relative c' {
+  \testFunc "test string"
+  c2 <c \testFunc "test string" e>
+}