t7001: use "ls-files --format" instead of "cut"

t7001: use "ls-files --format" instead of "cut"

Since ls-files recently learned a "--format" option, we can use that
rather than asking for all of "--stage" and then pulling out the bits we
want with "cut". That's simpler and avoids two extra processes (one for
cut, and one for the subshell to hold the intermediate result).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>