[tlbuild] pretest for tl21 starting early

[Karl Berry]

It seems that the source changes for TL21 are already pretty
stable. Thanks to all the developers for making that happen.

Thus I think we may as well start the TL21 pretest a few days early, on
the 24th or 25th, depending on how much flux there is. If the pretest
goes well, perhaps we can move up the rest of the schedule also, and
ultimately make the release a little earlier. (As always, I keep the
never-set-in-stone schedule on http://tug.org/texlive; not yet updated
for this possible change.)

If anyone knows of reasons not to start the pretest early, please let me
know asap.

Thanks,
Karl

P.S. This year, unlike all previous years, I plan to keep updating tlnet
(=> CTAN) with regular package updates even after the pretest
starts. We'll have to see how it goes, but that's the goal.  Of course,
I dearly hope people will still install and work with the pretest, since
that's the only way the new binaries will get any real-world testing.
More about this when the time comes.