p2000: add 'git checkout -' test and decrease depth

p2000: add 'git checkout -' test and decrease depth

As we increase our list of commands to test in
p2000-sparse-operations.sh, we will want to have a slightly smaller test
repository. Reduce the size by a factor of four by reducing the depth of
the step that creates a big index around a moderately-sized repository.

Also add a step to run 'git checkout -' on repeat. This requires having
a previous location in the reflog, so add that to the initialization
steps.

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