[MERGE] remove has_key() usage

[Aaron Bentley]

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Robey Pointer wrote:
> My no_has_key branch at:
> 
>     http://www.lag.net/~robey/code/bzr.dev.no_has_key/
> 
> has removed remaining usage of hasattr().

These sorts of changes do have collateral damage on long-lived branches,
like nested-trees.   They're worse than normal changes, because they
touch a large number of files, and normal changes are more focused.

It is somewhat frustrating dealing with these spurious changes in
addition to the functional changes.

If you're looking for janitorial work, can I suggest replacing the use
of None for the null revision with NULL_REVISION?  That would be a
highly useful, non-spurious change, because it would free us to use
"None" as "no revision supplied" or "no revision found".  Even if we
change NULL_REVISION to another name later, it's much easier to grep for
NULL_REVISION than to find the uses of None that mean "null revision".

Aaron
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