Fixes for Update Policy

Automatically merging remote changes into project repositories as part of a build is counter-productive; merges should be an explicit action rather than an implicit one.  This change lets the developer set the policy for git updates via <PROJECT>_REPO_UPDATE_POLICY.  The new default ("NONE") does nothing.  "REMOTE_UPDATE" is the one we use (this way we can find out if we are behind with a 'git status').  "FETCH_MERGE" is the old default.  "CMAKE_DEFAULT" is the other option.