FileVersions - change comparsion function

change time comparsion formula. we'd better have forumala
than can be spit to as f(v1) <=> f(v2), because we need a definition
of "version" than can be used in later commands directly.

Also I don't like the fact that one file without mtime force ignore
of all files' mtime (with same name)

Also previous function candidate was not transitive!
FileVersions - transitivity test