fixing sorting and improving dramatically restore diff backup:

fixing sorting and improving dramatically restore diff backup:

* sorting was not correct because the directory separator was being taken
  into account as part of the sorting, and thus if a directory contained
  a character like '-' it would affect sorting when compared with '/'.
  Now we sort directory by directory in a path, so the dir separator is
  not sorted itself.

* diffs backups where very slow because we were using a very simple
  algorithm that was also very slow, were for each file we would reopen
  and go across the index until found. Now we go across the index only
  once in the whole restore operation, so the speed of restoring a
  backup has increased dramatically.