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.
git_index_read_tree() was exposing a parameter to provide the user with
a progress indicator. Unfortunately, due to the recursive nature of the
tree walk, the maximum number of items to process was unknown. Thus,
the indicator was only counting processed entries, without providing
any information how the number of remaining items.
