Bugs with \set Staff.whichBar

[Ben Rudiak-Gould]

The 2.16 documentation says that \set Staff.whichBar = "|:" "will create a
start-repeat bar in this staff only". What it actually seems to do in 2.16.1 is
insert a start-repeat bar after every note in that staff until the end of the
score. I haven't looked at the code but I guess that it's clearing
Timing.whichBar even if whichBar was actually set in a different context.

The obvious workaround, \once \set Staff.whichBar = "|:", prints the repeat bar
correctly but then never prints any automatic bar lines in that staff until the
end of the piece. I think this may be a bug introduced by
codereview.appspot.com/4810042, which never unsets a \once \set variable even if
it was unset before.

-- Ben