grob.cc: rewrite O(n^2) algorithm in Grob::common_refpoint algorithm to O(n) (issue 5371050)

[joeneeman]

http://codereview.appspot.com/5371050/diff/1/lily/grob.cc
File lily/grob.cc (right):

http://codereview.appspot.com/5371050/diff/1/lily/grob.cc#newcode536
lily/grob.cc:536: c = c->dim_cache_[a].parent_;
After this loop, balance will be 0, right? So the next loop won't do
anything...

http://codereview.appspot.com/5371050/diff/1/lily/grob.cc#newcode542
lily/grob.cc:542: while (c != d) {
{ on new line

http://codereview.appspot.com/5371050/diff/1/lily/grob.cc#newcode542
lily/grob.cc:542: while (c != d) {
The old version would return 0 if there was no common refpoint (which is
possible if the grob hierarchy hasn't been fully built yet). I'd suggest
a "while (c && c != d)" to keep the old behavior.

http://codereview.appspot.com/5371050/