Code review comment for lp:~mbp/bzr/385103-format-name

This patch gets 2a ready for wider testing, and takes the position we will try for no more format changes before 2.0.

* adds a new format with an on-disk and command-line name of "2a"
* adds some text to NEWS about the format - comments on this are particularly welcome

We want somewhat wider testing but we're not yet confident that everyone should switch: we want to ramp up usage, and we still know of some cases that are slow. So I'm promoting this from development7

It would still be good to do this, but I haven't yet:

* add a special fast path for upgrading development7 to 2a - there are probably so few users that they can change it themselves
* stop --format creating special options for every format; it looks a bit awful