mv: check if out-of-cone file exists in index with SKIP_WORKTREE bit

mv: check if out-of-cone file exists in index with SKIP_WORKTREE bit

Originally, moving a <source> file which is not on-disk but exists in
index as a SKIP_WORKTREE enabled cache entry, "giv mv" command errors
out with "bad source".

Change the checking logic, so that such <source>
file makes "giv mv" command warns with "advise_on_updating_sparse_paths()"
instead of "bad source"; also user now can supply a "--sparse" flag so
this operation can be carried out successfully.

Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>