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

[Aaron Bentley]

Jelmer Vernooij wrote:
> I just fear that waiting for subtrees
> means it'll be another year before we're actually ready to make such a
> format the default.

I am not talking about waiting for the feature to be complete.  I am
talking about making sure that people don't get by-reference trees
unless they do so deliberately.  Once that's in place, we can make it
the default without requiring the feature to be polished.

Aaron