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.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes