rm: add --sparse option

rm: add --sparse option

As we did previously in 'git add', add a '--sparse' option to 'git rm'
that allows modifying paths outside of the sparse-checkout definition.
The existing checks in 'git rm' are restricted to tracked files that
have the SKIP_WORKTREE bit in the current index. Future changes will
cause 'git rm' to reject removing paths outside of the sparse-checkout
definition, even if they are untracked or do not have the SKIP_WORKTREE
bit.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>