This PS makes layering work for grandparents, not just parents. This
means that given a document with layer N, then not only can a parent
document in layer N+1 be used, but so can a grandparent in layer N+2.
Note that the document in layer N+1 will be preferred over the one
in N+2.
To provide a concrete example, given layers 'global', 'region'
and 'site', a document with layer 'site' can be layered with
a grandparent in layer 'global' if a document in layer
'region' isn't found. Alternatively, if the document with
layer 'region' is found then it will be used as the parent
document.
This PS refactors algorithm for determining the parent document,
tweaks the layering algorithm itself to reference the
correct parent document, and adds unit tests to confirm the
two scenarios described above.
Change-Id: I3779c7ae030bbad44b2d5ddfa5105d1a073ba670
9f7ecc0582