unpack-trees: ensure full index

The next change will translate full indexes into sparse indexes at write time. The existing logic provides a way for every sparse index to be expanded to a full index at read time. However, there are cases where an index is written and then continues to be used in-memory to perform further updates. unpack_trees() is frequently called after such a write. In particular, commands like 'git reset' do this double-update of the index. Ensure that we have a full index when entering unpack_trees(), but only when command_requires_full_index is true. This is always true at the moment, but we will later relax that after unpack_trees() is updated to handle sparse directory entries. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>