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