revset: lookup descendents for negative arguments to ancestor operator Negative offsets to the `~` operator now search for descendents. The search is aborted when a node has more than one child as we do not have a definition for 'nth child'. Optionally we can introduce such a notion and take the nth child ordered by rev number. The current revset language does provides a short operator for ancestor lookup but not for descendents. This gives user a simple revset to move to the previous changeset, e.g. `hg up '.~1'` but not to the 'next' changeset. With this change userse can now use `.~-1` as a shortcut to move to the next changeset. This fits better into allowing users to specify revisions via revsets and avoiding the need for special `hg next` and `hg prev` operations. The alternative to negative offsets is adding a new operator. We do not have many operators in ascii left that do not require bash escaping (',', '_', and '/' come to mind). If we decide that we should add a more convenient short operator such as ('/', e.g. './1') we can later add it and allow ascendents lookup via negative numbers.