[MERGE][1.7] Alert users when --stacked overrides format

[Aaron Bentley]

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John Arbash Meinel wrote:
> Your new code has a different path, which doesn't trigger this. Which is a bit
> sad, because I *do* think we should tell people when we use a branch format
> that isn't backwards compatible.

I think I completely missed that.  I agree that fixing it would be good.

> So I'm proposing the attached patch for bzr-1.7 which gives:
> $ bzr branch --stacked ~/dev/bzr/bzr.dev
> Stacking requires repository format:
>   Packs 5 (adds stacking support, requires bzr 1.6)
> Not preserving source format.

That's nice, but it doesn't actually say that Packs 5 is being used.

How about:
Because source format is not stacking-compatible, using format:
Packs 5 (adds stacking support, requires bzr 1.6)

bb:tweak

Aaron
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