t7519: rewrite sparse index test
The sparse index is tested with the FS Monitor hook and extension since
f8fe49e (fsmonitor: integrate with sparse index, 2021-07-14). This test
was very fragile because it shared an index across sparse and non-sparse
behavior. Since that expansion and contraction could cause the index to
lose its FS Monitor bitmap and token, behavior is fragile to changes in
'git sparse-checkout set'.
Rewrite the test to use two clones of the original repo: full and
sparse. This allows us to also keep the test files (actual, expect,
trace2.txt) out of the repos we are testing with 'git status'.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>