reset: fix validation in sparse index test
Update t1092 test 'reset with pathspecs outside sparse definition' to verify
index contents. The use of `rev-parse` verifies the contents of HEAD, not
the index, providing no real validation of the reset results. Conversely,
`ls-files` reports the contents of the index (OIDs, flags, filenames), which
are then compared across checkouts to ensure compatible index states.
Fixes
741a2c9ffa (reset: expand test coverage for sparse checkouts,
2021-09-27).
Signed-off-by: Victoria Dye <vdye@github.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>