t1092: behavior for adding sparse files

t1092: behavior for adding sparse files

Add some tests to demonstrate the current behavior around adding files
outside of the sparse-checkout cone. Currently, untracked files are
handled differently from tracked files. A future change will make these
cases be handled the same way.

Further expand checking that a failed 'git add' does not stage changes
to the index.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>