sparse-index: design doc and format update

This begins a long effort to update the index format to allow sparse
directory entries. This should result in a significant improvement to
Git commands when HEAD contains millions of files, but the user has
selected many fewer files to keep in their sparse-checkout definition.

Currently, the index format is only updated in the presence of
extensions.sparseIndex instead of increasing a file format version
number. This is temporary, and index v5 is part of the plan for future
work in this area.

The design document details many of the reasons for embarking on this
work, and also the plan for completing it safely.

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

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