Send only 1 flush pkt at the end of v2 fetch

When processing a fetch using protocol v2, UploadPack#fetchV2 sends an
extraneous flush pkt when also sending a packfile (#sendPack sending its
own flush pkt). Update that method to only send the flush pkt if the
packfile is not being sent.

Change-Id: I7117a264bccd2d7f3a048645fcb8425a9d78d526
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/UploadPackTest.java

@@ -658,6 +658,10 @@ public class UploadPackTest {
 				new StringWriter(), NullOutputStream.INSTANCE);
 		PackParser pp = client.newObjectInserter().newPackParser(sb);
 		pp.parse(NullProgressMonitor.INSTANCE);
+
+		// Ensure that there is nothing left in the stream.
+		assertThat(recvStream.read(), is(-1));
+
 		return pp.getReceivedPackStatistics();
 	}
 

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

@@ -1083,8 +1083,12 @@ public class UploadPack {
 						? db.getRefDatabase().getRefsByPrefix(R_TAGS)
 						: null,
 					unshallowCommits);
+			// sendPack invokes pckOut.end() for us, so we do not
+			// need to invoke it here.
+		} else {
+			// Invoke pckOut.end() by ourselves.
+			pckOut.end();
 		}
-		pckOut.end();
 	}
 
 	/*