t1092: test interesting sparse-checkout scenarios

t1092: test interesting sparse-checkout scenarios

These also document some behaviors that differ from a full checkout, and
possibly in a way that is not intended.

The test is designed to be run with "--run=1,X" where 'X' is an
interesting test case. Each test uses 'init_repos' to reset the full and
sparse copies of the initial-repo that is created by the first test
case. This also makes it possible to have test cases leave the working
directory or index in unusual states without disturbing later cases.

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