t2407: test branches currently using apply backend

t2407: test branches currently using apply backend

The tests in t2407 that verify the branch_checked_out() helper in the
case of bisects and rebases were added by 9347303db89 (branch: check for
bisects and rebases, 2022-06-08). However, that commit failed to check
for rebases that are using the 'apply' backend.

Add a test that checks the apply backend. The implementation was already
correct here, but it is good to have regression tests before modifying
the implementation further.

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