Fix while boundries in DateRevQueue.add()

In add(), "low" will never equals "first". This fact
should be reflected in the code.

Change-Id: I5cab51374e67bd2d3301e5d9dac47c4259b5e562

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/DateRevQueue.java

@@ -106,7 +106,7 @@ public class DateRevQueue extends AbstractRevQueue {
 				}
 			}
 			low = Math.min(low, high);
-			while (low >= first && when == index[low].commit.commitTime)
+			while (low > first && when == index[low].commit.commitTime)
 				--low;
 			q = index[low];
 		}