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(integer, string, string, integer, array) called at [<phabricator>/src/applications/diffusion/protocol/DiffusionCommandEngine.php:52]
#1 DiffusionCommandEngine::setURI(string) called at [<phabricator>/src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php:601]
...
```
Test Plan: Clusterized an observed repository, demoted a node, ran `bin/repository update Rxxx` to update, saw no typehint fatal.
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16390