Summary:
We are adding a simple transaction that works only with filelogs but
allows for concurrent access from multiple workers. This allows for a proper
rollback in case of a failure in a worker process, which previously would result
in bad data in the repositoriy.
Test Plan: rt test-p4* test-check*
Reviewers: #mercurial, durham, quark
Reviewed By: quark
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D5070340
Signature: t1:5070340:1494958313:b10b1eac5b42b36d1a587c4ae1c95fc2f8b5ad35