timeless@gmail.com [Fri, 26 Aug 2016 16:57:45 +0000] rev 2000
init: guard _namemap with repo.topics (issue5351)
This prevents reading extra data on all repo commits
when there is no topic for a given name.
_namemap is called a lot, and it is often called for commit
shas -- which should pretty much never be names of topics...