R1:48f53ba0951a

Allow updating diff with results for new unit tests

Summary:
When using postponed unittests to make 'arc diff' faster, there
are some situations where it is difficult to know exactly how
many unittests will be run. This is the case for many of our
C++ unittests, which we can't really know until we compile the
tests (which is slow, and probably isn't reasonable to be done
before posting the diff). I suppose we could make sure we
explicitly which tests a C++ unittest will run in some way, but
this would…