[MERGE] graph.heads() performance bugfix

[John Arbash Meinel]

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> As mentioned on IRC, one line not 2 here please.
> 
>> +                if key == 'deeper':
>> +                    import pdb;pdb.set_trace()
>> +                    self.fail('key deeper was accessed')
> 
> Hmm. When the tests pass, this 'import pdb;...' will never happen but we
> don't want it there in production code I assume.

I would probably agree, as it means bots auto-running the test suite will get
hung. Even though I have found it to be very useful when manually testing.
Places like this tend to help, in that the author of the test thought it would
be a good place to figure out what is going on. (A likely point of failure, etc.)

So we *might* consider just commenting it out, rather than deleting it.

John
=:->

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