Catch ArcanistUsageException when getting git parent log info

Summary:
ArcanistDiffWorkflow::getGitParentLogInfo() calls
ArcanistDifferentialCommitMessage::newFromRawCorpus() which may throw a
ArcanistUsageException if the parent commit message is malformed (specifically,
a bad "Differential Revision:" line); this should not stop arc diff.

Test Plan: successfully ran arc diff where the parent commit message was malformed.

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2434

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -996,6 +996,8 @@ EOTEXT
         }
       } catch (ArcanistDifferentialCommitMessageParserException $ex) {
         // Ignore.
+      } catch (ArcanistUsageException $ex) {
+        // Ignore an invalid Differential Revision field in the parent commit
       }
     }