t5520: ensure consistent test conditions

t5520: ensure consistent test conditions

Test title says that tests are done with rebase.autostash unset,
but does not take any action to make sure that it is indeed unset.
This may lead to test failure if future changes somehow pollutes
the configuration globally.

Ensure consistent test conditions by explicitly unsetting
rebase.autostash.

Signed-off-by: Mehul Jain <mehul.jain2029@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>