[MERGE] Add stable format names for stacked branches

[Aaron Bentley]

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

I don't think "stacked" is a good name choice, because it is the same as
the name of option to ``bzr branch`` and ``bzr push`` that causes the
result to be stacked.

Having ``bzr init --stacked`` mean something different from what ``bzr
branch --stacked`` means is confusing IMHO.

Aaron
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