[MERGE] Conditionalize format warnings

[Aaron Bentley]

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Aaron Bentley wrote:

> You can make base a required parameter of _check_supported, and issue a
> deprecation warning if it's not provided.

Actually, since it's internal, a warning isn't strictly necessary.

Oh, also: it looks like this is an infinite loop waiting to happen:

Branch.open => BzrDir.open_branch => BzrDir._check_supported => Branch.open

Aaron
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