ObjectDownloadListener#onWritePossible: Add comment on return statement

It is not obvious why this return statement is needed. Clarify with a
comment that otherwise endless loop may show up when recent versions
of Jetty are used.

Change-Id: I8e5d4de51869fb1179bf599bfb81bcd7d745874b
Signed-off-by: David Ostrovsky <david@ostrovsky.org>

org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/fs/ObjectDownloadListener.java

@@ -134,6 +134,10 @@ public class ObjectDownloadListener implements WriteListener {
 					} finally {
 						context.complete();
 					}
+					// This is need to avoid endless loop in recent Jetty versions.
+					// That's because out.isReady() is returning true for already
+					// closed streams and because out.close() doesn't throw any
+					// exception any more when trying to close already closed stream.
 					return;
 				}
 			}