sparse-index: improve advice message instructions

sparse-index: improve advice message instructions

When an on-disk sparse index is expanded to a full one, it could be
due to some worktree state that requires looking at file entries
hidden within sparse tree entries. This can be avoided if the
worktree is cleaned up and some other issues related to the index
state are resolved.

Expand the advice message to include all of these cases, since 'git
sparse-checkout clean' is not currently capable of handling all
cases.

In the future, we may improve the behavior of 'git sparse-checkout
clean' to handle all of the cases.

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