[MERGE] Add stable format names for stacked branches

[Martin Pool]

This adds new formats --format stacked and stacked-subtree for the branch
format that supports stacking, and also a new internal format marker.

The description printed by info on such branches is becoming way too long;
I'll work on making it describe and letting the user set each format 
individually.

I've used the class name in the on-disk format strings as that seems to 
me to tie it up in the most logical way.
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