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

[Aaron Bentley]

Martin Pool wrote:
> This
> 
>  * sets the default repository format to 1.9, which is straightforward
> but needs a few changes to blackbox tests that include the format
> number

If we're going to make a watershed, I'd prefer to switch to a
tree-reference format, so that we don't have another watershed and we
can stop having multiple variations of each repository format.

However, this requires some changes in the working tree, so that people
don't accidentally get nested trees.  Ideally, it would happen with a
new format.

However, I'd be able to start work on this next week.

However, it's a judgement call, and I'll work with whatever you decide.

Aaron