Code review comment for lp:~lifeless/bzr/bug-375013

Fix test_branch_from_trivial_stacked_branch_streaming_acceptance to work
with rich root formats, pending work on bug 375013.

Basically, committing to stacked branches is broken if there is any
actual content, but this test isn't trying to exercise that - and this
prevents changing the default in our test suite to a rich root format.

So I've fixed the test, and that may permit us to reduce the priority of
the bug (as its an existing defect in bzr, for any rich root format).

-Rob