stash: expand sparse-checkout compatibility testing

stash: expand sparse-checkout compatibility testing

Add tests verifying expected 'git stash' behavior in
't1092-sparse-checkout-compatibility'. These cases establish the expected
behavior of 'git stash' in a sparse-checkout and verify consistency both
with and without a sparse index. Although no sparse index compatibility has
been integrated into 'git stash' yet, the tests are all 'expect_success' -
we don't want the cone-mode sparse-checkout behavior to change depending on
whether it is using a sparse index or not. Therefore, we expect these tests
to continue passing once sparse index is integrated with 'git stash'.

Additionally, add performance test cases for 'git stash' both with and
without untracked files. Note that, unlike the other tests in
'p2000-sparse-operations.sh', the tests added for 'stash' are combination
operations. This is done to ensure the stash/unstash is not blocked by the
modification of '$SPARSE_CONE/a' performed as part of 'test_perf_on_all'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>