mv: refuse to move sparse paths

mv: refuse to move sparse paths

Since cmd_mv() does not operate on cache entries and instead directly
checks the filesystem, we can only use path_in_sparse_checkout() as a
mechanism for seeing if a path is sparse or not. Be sure to skip
returning a failure if '-k' is specified.

To ensure that the advice around sparse paths is the only reason a move
failed, be sure to check this as the very last thing before inserting
into the src_for_dst list.

The tests cover a variety of cases such as whether the target is tracked
or untracked, and whether the source or destination are in or outside of
the sparse-checkout definition.

Helped-by: Matheus Tavares Bernardino <matheus.bernardino@usp.br>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>