staging vs. staging-broken-dec23

[Graham Percival]

Hey all,

In order to remove the blockage to development, I've made a copy
of the previous staging branch: it's now called
staging-broken-dec23.  I've replaced staging with
master+release/unstable, and Patchy is testing that right now.

Once that's been accepted to master, people are invited to
cherry-pick their patches from staging-broken-dec23 to staging if
you are very confident that they didn't cause the breakage.  I
don't think that anybody wants to get into huge build-system
surgery right over Christmas, so if you have any doubts, please
hold off unless you've tested a full doc build from scratch.

Doubts include:
- anything that touches the build system
- anything that adds a new file to Documentation/snippets/*.ly or
  input/regression/*.ly

Patchy is running every 6 hours at the moment (GMT / 6), and
there's no rush to get another release out, so let's just put
stuff into staging gradually.

Cheers,
- Graham