Keep at most two versions of the tree.
Keep at most two versions of the tree.

When modifying the tree, do not keep the intermediate versions around
but only the very original one and the new one.

	* src/ast/ast.yml (Ast::original_set): Reuse the "original" field.

urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi