[1.13][merge] set default format to 1.9 and tweak remote branch stacking

[Martin Pool]

1.9 is a straightforward improvement on packs-0.92; it is faster,
supports stacking (therefore avoids warnings about upgrading it) etc.

I originally intended in this branch to change to 1.9-rich-root.
There might still be things in that direction (John found one) and
that would be a fairly serious bug if we merged it.

That said this is now too late for 1.13rc1.

I do think in future we should probably advance the format in one or
two releases after it's published so that people can more easily get
good performance, and so that we find any small latent bugs that are
like this only shown when you run the test suite with the new format
as the default.

I'll try to fix them up and we can merge it for 1.14.

-- 
Martin <http://launchpad.net/~mbp/>