apply: integrate with the sparse index

The sparse index allows storing directory entries in the index, marked
with the skip-wortkree bit and pointing to a tree object. This may be an
unexpected data shape for some implementation areas, so we are rolling
it out incrementally on a builtin-per-builtin basis.

This change enables the sparse index for 'git apply'. The main
motivation for this change is that 'git apply' is used as a child
process of 'git add -p' and expanding the sparse index for each of those
child processes can lead to significant performance issues.

The good news is that the actual index manipulation code used by 'git
apply' is already integrated with the sparse index, so the only product
change is to mark the builtin as allowing the sparse index so it isn't
inflated on read.

The more involved part of this change is around adding tests that verify
how 'git apply' behaves in a sparse-checkout environment and whether or
not the index expands in certain operations.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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