sparse-index: point users to new 'clean' action

sparse-index: point users to new 'clean' action

In my experience, the most-common reason that the sparse index must
expand to a full one is because there is some leftover file in a tracked
directory that is now outside of the sparse-checkout. The new 'git
sparse-checkout clean' command will find and delete these directories,
so point users to it when they hit the sparse index expansion advice.

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