Make make-relative able to deal with music rather than just pitches (issue 30890043)

[pkx166h]

https://codereview.appspot.com/30890043/diff/1/input/regression/make-relative-music.ly
File input/regression/make-relative-music.ly (right):

https://codereview.appspot.com/30890043/diff/1/input/regression/make-relative-music.ly#newcode8
input/regression/make-relative-music.ly:8: well inside and outside of
@code{\\relative}."
This causes a make doc fail

--snip--
./0b/lily-d78c3e03.texidoc:2: Undefined control sequence.
l.2 function calls behave @q
                            {as expected} from the function's arguments
?
./0b/lily-d78c3e03.texidoc:2: Emergency stop.
l.2 function calls behave @q
                            {as expected} from the function's arguments
./0b/lily-d78c3e03.texidoc:2:  ==> Fatal error occurred, no output PDF
file pro
duced!
Transcript written on collated-files.log.
/usr/bin/texi2dvi: pdfetex exited with bad status, quitting.
--snip--

James

https://codereview.appspot.com/30890043/