sparse-checkout: check for dirty status

The index-merge performed by 'git sparse-checkout' will erase any staged
changes, which can lead to data loss. Prevent these attempts by requiring
a clean 'git status' output.

Helped-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
