sparse-checkout: hold pattern list in index

sparse-checkout: hold pattern list in index

As we modify the sparse-checkout definition, we perform index operations
on a pattern_list that only exists in-memory. This allows easy backing
out in case the index update fails.

However, if the index write itself cares about the sparse-checkout
pattern set, we need access to that in-memory copy. Place a pointer to
a 'struct pattern_list' in the index so we can access this on-demand.
This will be used in the next change which uses the sparse-checkout
definition to filter out directories that are outside the sparse cone.

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