t6601: tag otherwise-unreachable trees

t6601: tag otherwise-unreachable trees

The tests in t6601-path-walk.sh demonstrate the behavior of the
path-walk API under different conditions. One thing that I noticed while
updating the behavior of directly-requested objects is that we don't
actually emit tagged trees. This was previously not noticed due to those
tagged trees actually being reachable from commits that we are including
in the path-walk.

Update the test setup to have tree-tag and tree-tag2 point to trees that
are otherwise unreachable.

It is worth noting that this does not meaningfully change any of the
other test cases, demontrating the bug.

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