hg: prepare for Mercurial 5.0 changing "exact" arguments In the backward compat wrapper, we use root=None. That might seem a bit risky. But it seems to work for the single use case we have, and the changeset dropped it in Mercurial https://www.mercurial-scm.org/repo/hg/rev/0531dff73d0b hint that this parameter really is unused.