Nested contexts - fix

[Rune Zedeler]

Back in 2001 I made a patch to allow for nested StaffGroups. 
Unfortunately the introduced cycle in the context-hierarchy introduced a 
infinite loop in lilypond, leading to a stack overflow.
I therefore replaced the nested StaffGroups with StaffGroup and 
InnerStaffGroup - as some sort of temporary solution. But here, after 6 
years, they are still there.
Tried to push patch to my branch in git, but as stated in other mail, I 
have git problems.
So I attached patch instead.

Afaics the attached patch fixes the simple "Context pointing directly to 
itself" version of the problem (i.e. it does NOT fix A->B->A style 
loops). I think that applying this patch will allow for StaffGroups to 
accept StaffGroups - so the we can discontinue use of InnerStaffGroup 
and InnerChoirStaff for anything but staffgroups containing choirstaves 
and vice verca.

-Rune

diff --git a/lily/context-def.cc b/lily/context-def.cc
index f4023a0..760547f 100644
--- a/lily/context-def.cc
+++ b/lily/context-def.cc
@@ -223,13 +223,16 @@ Context_def::path_to_acceptable_context (SCM type_sym, 
Output_def *odef) const
     {
       Context_def *g = accepteds[i];
 
-      vector<Context_def*> result
-       = g->path_to_acceptable_context (type_sym, odef);
-      if (result.size () && result.size () < best_depth)
+      if (g!=this)
        {
-         best_depth = result.size ();
-         result.insert (result.begin (), g);
-         best_result = result;
+         vector<Context_def*> result
+           = g->path_to_acceptable_context (type_sym, odef);
+         if (result.size () && result.size () < best_depth)
+           {
+             best_depth = result.size ();
+             result.insert (result.begin (), g);
+             best_result = result;
+           }
        }
     }