t1092: add tests for `git diff-files`

t1092: add tests for `git diff-files`

Before integrating the 'git diff-files' builtin with the sparse index
feature, add tests to t1092-sparse-checkout-compatibility.sh to ensure
it currently works with sparse-checkout and will still work with sparse
index after that integration.

When adding tests against a sparse-checkout definition, we test two
modes: all changes are within the sparse-checkout cone and some changes
are outside the sparse-checkout cone.

In order to have staged changes outside of the sparse-checkout cone,
make a directory called 'folder1' and copy `a` into 'folder1/a'.
'folder1/a' is identical to `a` in the base commit. These make
'folder1/a' in the index, while leaving it outside of the
sparse-checkout definition. Change content inside 'folder1/a' in order
to test 'folder1/a' being present on-disk with modifications.

Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>