[patch][0.15] better DirState._validate

[Martin Pool]

Following on from John's move_children_correctly here is a patch that:

 * updates DirState._validate to be more correct
 * adds some tests for different types of incorrectness that we can detect
 * changes assertRaisess to return the exception object, so that we
can examine it
 * moves some test-creation code into the test_dirstate base class so
that it can be used by more tests

Since John's fix needs to be merged for 0.15 but it's validation
changes I want to put this in for the rc3, but review would be very
welcome.

-- 
Martin
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