Fixed issue #286

List commit API (https://developer.github.com/v3/repos/commits/#list-commits-on-a-repository) already populates short info, and so populate() call could be excessive.

 It's possible that the short info is always available and therefore there's never a need to call populate(), but that assumption is hard to test, so I'm leaving that in

src/main/java/org/kohsuke/github/GHCommit.java

@@ -179,7 +179,8 @@ public class GHCommit {
 
 
     public ShortInfo getCommitShortInfo() throws IOException {
-        populate();
+        if (commit==null)
+            populate();
         return commit;
     }