sparse-index: silently return when not using cone-mode patterns

sparse-index: silently return when not using cone-mode patterns

While the sparse-index is only enabled when core.sparseCheckoutCone is
also enabled, it is possible for the user to modify the sparse-checkout
file manually in a way that does not match cone-mode patterns. In this
case, we should refuse to convert an index into a sparse index, since
the sparse_checkout_patterns will not be initialized with recursive and
parent path hashsets.

Also silently return if there are no cache entries, which is a simple
case: there are no paths to make sparse!

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>