rm: honor sparse checkout patterns

rm: honor sparse checkout patterns

`git add` refrains from adding or updating index entries that are
outside the current sparse checkout, but `git rm` doesn't follow the
same restriction. This is somewhat counter-intuitive and inconsistent.
So make `rm` honor the sparsity rules and advise on how to remove
SKIP_WORKTREE entries just like `add` does. Also add some tests for the
new behavior.

Suggested-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>