test-reach: test get_reachable_subset

test-reach: test get_reachable_subset

The get_reachable_subset() method returns the list of commits in
the 'to' array that are reachable from at least one commit in the
'from' array. Add tests that check this method works in a few
cases:

1. All commits in the 'to' list are reachable. This exercises the
   early-termination condition.

2. Some commits in the 'to' list are reachable. This exercises the
   loop-termination condition.

3. No commits in the 'to' list are reachable. This exercises the
   NULL return condition.

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