Add ReftableNumbersNotIncreasingException#toString

Change-Id: I28b6a6476df185f3f98f8020408b4b120086cb0b
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileReftableStack.java

@@ -145,6 +145,14 @@ public class FileReftableStack implements AutoCloseable {
 			this.lastMax = lastMax;
 			this.min = min;
 		}
+
+		@SuppressWarnings({ "nls", "boxing" })
+		@Override
+		public String toString() {
+			return String.format(
+					"ReftableNumbersNotIncreasingException %s: min %d, lastMax %d",
+					name, min, lastMax);
+		}
 	}
 
 	/**