advice.h: add advise_on_moving_dirty_path()

advice.h: add advise_on_moving_dirty_path()

Add an advice.

When the user use `git mv --sparse <dirty-path> <destination>`, Git
will warn the user to use `git add --sparse <paths>` then use
`git sparse-checkout reapply` to apply the sparsity rules.

Add a few lines to previous "move dirty path" tests so we can test
this new advice is working.

Suggested-by: Derrick Stolee <redacted>
Signed-off-by: Shaoxuan Yuan <redacted>
Signed-off-by: Junio C Hamano <redacted>