simplify delta of file id, for multi-versioning of a file
simplify delta of file id, for multi-versioning of a file

git-svn-id: a5b205d308d305c7e93234f1073de5c192ae1c9b@54 282b0af5-e82d-9cf1-ede4-77906d7719d0

k85169336/seaweedfs,chrislusf/seaweedfs,hxiaodon/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs,hxiaodon/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs