Martin von Zweigbergk <martinvonz@google.com> [Tue, 22 Aug 2017 10:59:19 -0700] rev 2851
obscache: use _readmarkers() from core with correct signature
The copied _readmarkers() went stale with Mercurial core commit
5d3ba4395288 (obsstore: let read marker API take a range of offsets,
2017-06-04). At the same time, the Mercurial core version of the
function gained the desired offset argument, so we can now use that
function.