At the moment, it is only possible to merge covdbs which share the same top-level architecture. Merging coverage databases with different top-level architectures results in only the coverage from the first database being kept.

However, it is common to instantiate architectures in a hierarchy. This can result in misleading coverage results. For example, it could appear that A.B did not have sufficient coverage, even though there is a separate test for B which includes paths not covered in the test of A.B.

We should add support for merging covdbs with different top-level architectures, allowing a coverage summary to be created and easing identification of areas not covered under any test.