t5620: make test work with path-walk var

t5620: make test work with path-walk var

The GIT_TEST_PACK_PATH_WALK test variable allows enabling the
--path-walk option to 'git pack-objects' by default. This sometimes
engages the warning that --path-walk is incompatible with the --filter
option. These tests in t5620 fail due to this warning over stderr in
this case. Disable this variable for this moment until these options
work together.

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