t1092: add sparse directory before cone in test repo

t1092: add sparse directory before cone in test repo

Add a sparse directory 'before/' containing files 'a' and 'b' to the test
repo used in 't/t1092-sparse-checkout-compatibility.sh'. This is meant to
ensure that no sparse index integrations rely on the in-cone path(s) being
lexicographically first in the repo.

Unfortunately, some existing tests do not handle this repo architecture
properly:

* 'add outside sparse cone'
* 'status/add: outside sparse cone'
* 'reset with pathspecs inside sparse definition'

All three of these are due to the incorrect handling of the
'unpack_trees_options.cache_bottom' when performing a cache diff via
'unpack_trees'. This will be corrected in a future patch; in the meantime,
mark the tests with 'test_expect_failure'.

Finally, update the 'ls-files' and 'root directory cannot be sparse' tests
to include the 'before/' directory in their expected index contents.

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