Homestyx hydra

R1:39d4e21eec56

Fix a bad DiffusionCommandEngine parameter from HTTPEngine conversion

Summary:
I converted this call incorrectly in D16092. We should pass the `PhutilURI` object, not the string version of it.

Specifically, this resulted in hitting an error like this if a replica needed synchronization:

```
[2016-08-11 21:22:37] EXCEPTION: (InvalidArgumentException) Argument 1 passed to DiffusionCommandEngine::setURI() must be an instance of PhutilURI, string given, called in...
#0 PhutilErrorHandler::handleError…
Repository: R1 hydra
Commit Date: Aug 11 2016