Summary:
Now that all our repos are treemanifest, let's enable the extension by
default in tests. Once we're certain no one needs it in production we'll also
make it the default in core Mercurial.
This diff includes a minor fix in treemanifest to be aware of always-enabled
extensions. It won't matter until we actually add treemanifest to the list of
default enabled extensions, but I caught this while testing things.
Reviewed By: ikostia
Differential Revision: D15030253
fbshipit-source-id: d8361f915928b6ad90665e6ed330c1df5c8d8d86