revset: reintroduce and experimental revset for update destination The revset is not ready for prime time yet. However it is useful to have some version of it exposed to help candidate users to play with it and provide feedback on what we should aim at. We add a small test to make sure the code runs.

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.