Allow PreUploadHook.onSendPack to send messages to the client

Before transmitting to the client a hook may want to send along
a text message ahead of the pack, such as a "message of the day".
Enable this usage by mirroring the message sending API from
ReceivePack on the UploadPack instance, using the side band.

Change-Id: I31cd254a4ddb816641397a3e9c2c20212471c37f

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

 import org.eclipse.jgit.storage.pack.PackConfig;
 import org.eclipse.jgit.transport.BasePackFetchConnection.MultiAck;
 import org.eclipse.jgit.transport.RefAdvertiser.PacketLineOutRefAdvertiser;
+import org.eclipse.jgit.util.io.DisabledOutputStream;
 import org.eclipse.jgit.util.io.InterruptTimer;
 import org.eclipse.jgit.util.io.TimeoutInputStream;
 import org.eclipse.jgit.util.io.TimeoutOutputStream;
 
 	private PacketLineOut pckOut;
 
+	private OutputStream msgOut = DisabledOutputStream.INSTANCE;
+
 	/** The refs we advertised as existing at the start of the connection. */
 	private Map<String, Ref> refs;
 
 		try {
 			rawIn = input;
 			rawOut = output;
+			if (messages != null)
+				msgOut = messages;
 
 			if (timeout > 0) {
 				final Thread caller = Thread.currentThread();
 			pckOut = new PacketLineOut(rawOut);
 			service();
 		} finally {
+			msgOut = DisabledOutputStream.INSTANCE;
 			walk.release();
 			if (timer != null) {
 				try {
 		adv.end();
 	}
 
+	/**
+	 * Send a message to the client, if it supports receiving them.
+	 * <p>
+	 * If the client doesn't support receiving messages, the message will be
+	 * discarded, with no other indication to the caller or to the client.
+	 *
+	 * @param what
+	 *            string describing the problem identified by the hook. The
+	 *            string must not end with an LF, and must not contain an LF.
+	 */
+	public void sendMessage(String what) {
+		try {
+			msgOut.write(Constants.encode(what + "\n")); //$NON-NLS-1$
+		} catch (IOException e) {
+			// Ignore write failures.
+		}
+	}
+
+	/** @return an underlying stream for sending messages to the client, or null. */
+	public OutputStream getMessageOutputStream() {
+		return msgOut;
+	}
+
 	private void recvWants() throws IOException {
 		boolean isFirst = true;
 		for (;;) {
 	private void sendPack(final boolean sideband) throws IOException {
 		ProgressMonitor pm = NullProgressMonitor.INSTANCE;
 		OutputStream packOut = rawOut;
-		SideBandOutputStream msgOut = null;
 
 		if (sideband) {
 			int bufsz = SideBandOutputStream.SMALL_BUF;
 			pw.writePack(pm, NullProgressMonitor.INSTANCE, packOut);
 			statistics = pw.getStatistics();
 
-			if (msgOut != null) {
+			if (msgOut != DisabledOutputStream.INSTANCE) {
 				String msg = pw.getStatistics().getMessage() + '\n';
 				msgOut.write(Constants.encode(msg));
 				msgOut.flush();