sparse-index: recompute cache-tree

sparse-index: recompute cache-tree

When some commands run with command_requires_full_index=1, then the
index can get in a state where the in-memory cache tree is actually
equal to the sparse index's cache tree instead of the full one.

This results in incorrect entry_count values. By clearing the cache
tree before converting to sparse, we avoid this issue.

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