t1092: stress test 'git sparse-checkout set'

t1092: stress test 'git sparse-checkout set'

The 'sparse-index contents' test checks that the sparse index has the
correct set of sparse directories in the index after modifying the cone
mode patterns using 'git sparse-checkout set'. Add to the coverage here
by adding more complicated scenarios that were not previously tested.

In order to check paths that do not exist at HEAD, we need to modify the
test_sparse_checkout_set helper slightly:

1. Add the --skip-checks argument to the 'set' command to avoid failures
   when passing paths that do not exist at HEAD.

2. When looking for the non-existence of sparse directories for the
   paths in $CONE_DIRS, allow the rev-list command to fail because the
   path does not exist at HEAD.

This allows us to add some interesting test cases.

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