some work towards incremental backups

git-svn-id: https://plugins.svn.wordpress.org/updraftplus/trunk@942279 b8457f37-d9ea-0310-8a92-e5e31aec5664
some work towards incremental backups

git-svn-id: https://plugins.svn.wordpress.org/updraftplus/trunk@942284 b8457f37-d9ea-0310-8a92-e5e31aec5664