Create and use a test-specific branch when the test might create a
commit. This is not always necessary for correctness, but it improves
debuggability by ensuring a commit created by test #N shows up on the
testN branch, not the branch for test #N-1.
Signed-off-by: Richard Hansen <hansenr@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>