Fix a view fatal in CommitGraphView when commits are undiscovered

Summary:
Ref T13552. See <https://discourse.phabricator-community.org/t/viewing-repository-history-for-svn-repository-causes-unhandled-exception/4225/>.

This condition is flipped and can fatal by passing a `NULL` value for `$commit` to a typehinted method.

Test Plan: Viewed history page with undiscovered commits.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21458

src/applications/diffusion/view/DiffusionCommitGraphView.php

@@ -485,7 +485,7 @@ final class DiffusionCommitGraphView
     $buildable = null;
 
     $commit = $this->getCommit($hash);
-    if (!$commit) {
+    if ($commit) {
       $buildable = $this->getBuildable($commit);
     }