run-tests: add a way to list tests, with JSON and XUnit support Some test runners are interested in listing tests, so they can do their own filtering on top (usually based on attributes like historically observed runtime). Add support for that.

run-tests: install hg after computing tests to run We're going to add a way to list tests, and we don't need to install hg for that.

run-tests: make time field optional for xunit report We're going to use XUnit to list tests, and we don't have a time field in that case.

run-tests: factor out json write code into another method We're going to use this code to output a JSON-formatted listing of tests.

run-tests: factor out xunit write code into another method We're going to use this code to output an XUnit-formatted listing of tests.

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.

update: show the commit to which we updated in case of multiple heads (BC) Currently when we have multiple heads on the same branch, update tells us that there some more heads for the current branch but does not tells us the head to which the repository has been updated to. It makes more sense showing the head we updated to and then telling there are some more heads.