stop pushing to master (contextStringTuning is broken)

[Graham Percival]

Guys,
  ----------------------------------------------
  ***  STOP PUSHING BROKEN STUFF TO MASTER.  ***
  ----------------------------------------------

At least until we've had two more unstable releases (.9 and .10)
If you haven't tested your patch from a COMPLETELY EMPTY build
directory, done make and make docs FROM SCRATCH, then don't push
at all.


%%% from fretted-strings:
mynotes = {
  c'4 e' g' c'' |
  e''4 g'' b'' c'''
}
<<
  \new Staff {
    \clef treble
    \mynotes
  }
  \new TabStaff {
    \contextStringTuning #'custom-tuning <c' g' d'' a''>
    \mynotes
  }
>>


%%% produces:
<string>:1:20: error: syntax error, unexpected '(', expecting '='
 \set stringTunings 
                    (ly:parser-lookup \lilyvartmpi \lilyvartmpj)
foo.ly:13:4: error: Music head function must return Music object
    
    \contextStringTuning #'custom-tuning <c' g' d'' a''>


I'm going to revert 2dad7be345ca1f4c04e836a56a9a66c0e340b39b
if that fails, I'll revert
d8ee829c73452cc35ef17c6c22abf5b47c754c15 , and if that still
fails, I'll start randomly reverting stuff until git master
compiles again.

- Graham