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.