2.13.12 out; please check things

[Graham Percival]

As discussed on:

http://lilypond.org/doc/v2.13/Documentation/contributor/checking-and-verifying-issues#Checking-and-verifying-issues

Please check the regtest comparison, and verify any items that you can
verify.  If you don't feel confident about judging an issue, leave it
alone.  In a few days, I'll go through and check any non-verified
fixed items.  But it would be nice if all the obvious stuff was dealt
with, so that I can spend more time on other matters.

Cheers,
- Graham