Don't create a snapshot in Initialize(), but leave the creation
responsibility to the cmake instance instead.  Previously, the
cmState would Initialize() in its constructor, and the cmake instance
would re-Initialize() during Configure().  The end result was the
same and there would be one snapshot present.  However, cmLocalGenerator
also created a snapshot on construction, and that one was used, leaving
the first snapshot unused, and potential for off-by-one errors.

Fix that by making the cmLocalGenerator use the existing snapshot
if it is top-level.  Add a CurrentSnapshot to the cmake instance and
populated it while configuring a directory.  This will eventually
replace the 'current local generator' concept.  Fix the GetParent
implementation to be able to return the first snapshot.