Fix push option initalization on HTTP

Initialize pushOptions when we decide to use them, instead of when we
advertise them.

In the case of HTTP the advertisement is in a different network
request, hence in a different instance of the BaseReceivePack.

Change-Id: I094c60942e04de82cb6d8433c9cd43a46ffae332
Signed-off-by: Stefan Beller <sbeller@google.com>

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

@@ -220,7 +220,7 @@ public class PushOptionsTest extends RepositoryTestCase {
 				one.getStatus());
 		assertSame(RemoteRefUpdate.Status.REJECTED_REMOTE_CHANGED,
 				two.getStatus());
-		assertEquals(new ArrayList<String>(), baseReceivePack.getPushOptions());
+		assertNull(baseReceivePack.getPushOptions());
 	}
 
 	@Test

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

@@ -1152,7 +1152,6 @@ public abstract class BaseReceivePack {
 			adv.advertiseCapability(CAPABILITY_OFS_DELTA);
 		if (allowPushOptions) {
 			adv.advertiseCapability(CAPABILITY_PUSH_OPTIONS);
-			pushOptions = new ArrayList<>();
 		}
 		adv.advertiseCapability(OPTION_AGENT, UserAgent.get());
 		adv.send(getAdvertisedOrDefaultRefs());
@@ -1272,6 +1271,9 @@ public abstract class BaseReceivePack {
 		quiet = allowQuiet && isCapabilityEnabled(CAPABILITY_QUIET);
 		usePushOptions = allowPushOptions
 				&& isCapabilityEnabled(CAPABILITY_PUSH_OPTIONS);
+		if (usePushOptions) {
+			pushOptions = new ArrayList<>();
+		}
 		if (sideBand) {
 			OutputStream out = rawOut;