Update git to v1.8.2.1

This requires a small change to how we handle notes, but otherwise just
works.

Note that we can't use anything from v1.8.0 until v1.8.2.1 because some
of the symbols that we need for graph drawing were made private in
v1.8.0 and this was not reverted until v1.8.2.1.

Signed-off-by: John Keeping <john@keeping.me.uk>