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 require a lot of change to our backend test infra.
Also, it seems that this is a pretty general issue of not knowing
how many unittests will be run until they actually run.

This diff adds an optional "create" parameter to updateunitresults
which wil create a new unit tests result rather than updating an
existing one. I am not sure if this really fits here or should
be its own method, but there is a lot of code re-use between them
so I consolidated.

Test Plan: updated a diff with a new unit test result

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley, andrewjcg, tuomaspelkonen

Differential Revision: https://secure.phabricator.com/D1352