overlayworkingctx: add _auditconflicts to write()
Alas, part of Mercurial's conflict detection (for file<->folder conflicts,
for example) depends on the filesystem. We don't have the filesystem with IMM,
so we have to run these checks ourselves.
Differential Revision: https://phab.mercurial-scm.org/D1241
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh