A number of diff APIs and the `git_checkout_index` API take a
`git_repository` object an operate on the index. This updates
them to take a `git_index` pointer explicitly and only fall back
on the `git_repository` index if the index input is NULL. This
makes it easier to operate on a temporary index.
This is a major reworking of checkout strategy options. The
checkout code is now sensitive to the contents of the HEAD tree
and the new options allow you to update the working tree so that
it will match the index content only when it previously matched
the contents of the HEAD. This allows you to, for example, to
distinguish between removing files that are in the HEAD but not
in the index, vs just removing all untracked files.
Because of various corner cases that arise, etc., this required
some additional capabilities in rmdir and other utility functions.
This includes the beginnings of an implementation of code to read
a partial tree into the index based on a pathspec, but that is
not enabled because of the possibility of creating conflicting
index entries.
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.
