[MERGE] Restore default stacking policy support

[Aaron Bentley]

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John Arbash Meinel wrote:
> Aaron Bentley wrote:
> ^- ``bzr branch`` uses the default stacking policy if the branch format
>    supports it.
> 
> Perhaps?

Sure.

> +    def _get_metadir(self, require_stacking=False):
> 
> ^- I'm not sure about the name of this function. It doesn't seem clear what
> the difference is between this and "cloning_metadir". Is there a reason to not
> just update the api of "cloning_metadir" to take a "require_stacking" parameter?

Just the burden on BzrDir implementors.  I think I'll do it anyhow,
though.  I don't like it much, either.

> ^- I realize this is somewhat necessary, but it does feel a bit odd to be
> hard-coding Format7. I know we have the bug open about 'subtree/rich-root',
> but as we add more branch formats it seems like we have to keep updating this.
> Would it be possible to have a single recommended-for-stacking registry rather
> than hard-coding this? (This is not a blocking issue, this is just being
> raised for discussion.)

If it ever turns out we want to vary this, I'll be happy to create some
kind of registry.  But for now, I'd like to leave it in.  It should stop
mattering as the stacking-capable formats become common.

Aaron
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