UploadPack: advertise allow-tip-sha1-in-want

This protocol capability, new in C git 1.8.2, corresponds to
RequestPolicy.TIP, so advertise it if that request policy was set.

Change-Id: I0d52af8a7747e951a87f060a5124f822ce1b2b26

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

@@ -181,6 +181,13 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 	 */
 	public static final String OPTION_NO_DONE = "no-done"; //$NON-NLS-1$
 
+	/**
+	 * The client supports fetching objects at the tip of any ref, even if not
+	 * advertised.
+	 * @since 3.1
+	 */
+	public static final String OPTION_ALLOW_TIP_SHA1_IN_WANT = "allow-tip-sha1-in-want"; //$NON-NLS-1$
+
 	static enum MultiAck {
 		OFF, CONTINUE, DETAILED;
 	}

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

@@ -90,6 +90,8 @@ import org.eclipse.jgit.util.io.TimeoutOutputStream;
  * Implements the server side of a fetch connection, transmitting objects.
  */
 public class UploadPack {
+	static final String OPTION_ALLOW_TIP_SHA1_IN_WANT = BasePackFetchConnection.OPTION_ALLOW_TIP_SHA1_IN_WANT;
+
 	static final String OPTION_INCLUDE_TAG = BasePackFetchConnection.OPTION_INCLUDE_TAG;
 
 	static final String OPTION_MULTI_ACK = BasePackFetchConnection.OPTION_MULTI_ACK;
@@ -122,8 +124,8 @@ public class UploadPack {
 		REACHABLE_COMMIT,
 
 		/**
-		 * Client may ask for objects that are the tip of some reference, even if
-		 * that reference wasn't advertised.
+		 * Client may ask for objects that are the tip of any reference, even if not
+		 * advertised.
 		 * <p>
 		 * This may happen, for example, when a custom {@link RefFilter} is set.
 		 */
@@ -722,6 +724,9 @@ public class UploadPack {
 		adv.advertiseCapability(OPTION_SHALLOW);
 		if (!biDirectionalPipe)
 			adv.advertiseCapability(OPTION_NO_DONE);
+		if (requestPolicy == RequestPolicy.TIP
+				|| requestPolicy == RequestPolicy.REACHABLE_COMMIT_TIP)
+			adv.advertiseCapability(OPTION_ALLOW_TIP_SHA1_IN_WANT);
 		adv.setDerefTags(true);
 		advertised = adv.send(getAdvertisedOrDefaultRefs());
 		adv.end();