New visitor method
New iterative methods (cloning still pending and validation still buggy)
Fixing :validate bug and broken tests
Implementing iterative clone method
Minor changes...
Changing collation of db_filenames table so case comparisons are sensitive
Fixing ORDER BY clause, by including the table alias
Replacing 'parents_sha1' by 'parents_sha1s'
Finishing 1st working version of iterative :clone_into