Fix checkout behavior when its hands are tied

So, @nulltoken created a failing test case for checkout that
proved to be particularly daunting.  If checkout is given only
a very limited strategy mask (e.g. just GIT_CHECKOUT_CREATE_MISSING)
then it is possible for typechange/rename modifications to leave it
unable to complete the request.  That's okay, but the existing code
did not have enough information not to generate an error (at least
for tree/blob conflicts).

This led me to a significant reorganization of the code to handle
the failing case, but it has three benefits:

1. The test case is handled correctly (I think)
2. The new code should actually be much faster than the old code
   since I decided to make checkout aware of diff list internals.
3. The progress value accuracy is hugely increased since I added
   a fourth pass which calculates exactly what work needs to be
   done before doing anything.

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