Pass full Harbormaster URIs to Buildkite

Summary: See PHI611 for details.

Test Plan:
Ran a Buildkite build, saw Buildkite confirm receipt of these parameters in the HTTP response:

{F5562054}

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D19419

src/applications/harbormaster/step/HarbormasterBuildkiteBuildStepImplementation.php

@@ -106,6 +106,14 @@ EOTEXT
       ),
       'meta_data' => array(
         'buildTargetPHID' => $build_target->getPHID(),
+
+        // See PHI611. These are undocumented secret magic.
+        'phabricator:build:id' => (int)$build->getID(),
+        'phabricator:build:url' =>
+          PhabricatorEnv::getProductionURI($build->getURI()),
+        'phabricator:buildable:id' => (int)$buildable->getID(),
+        'phabricator:buildable:url' =>
+          PhabricatorEnv::getProductionURI($buildable->getURI()),
       ),
     );