Return changes from mergeable in ChangeJson only if tested

If background mergeability checks are not working for some reason, we
should not blindly report that changes are unmergeable. We have enough
information stored in the change, namely the last SHA-1 tested
against, to know whether a check was ever successfully run. Take
advantage of this in ChangeJson.

Change-Id: I4df800d66ac929e9cd152795acd9f5b671bfd101
2 files changed