PullCommandTest#testPullConflictDiff3: assert pull result

to fix errorprone error "UnusedVariable"

```
$ bazelisk test //...
...
  Did you mean 'assertEquals(MergeStatus.CONFLICTING,' or
'assertEquals(/* expected= */res.getMergeResult().getMergeStatus(),'?
org.eclipse.jgit.test/tst/org/eclipse/jgit/api/PullCommandTest.java:191:
error: [UnusedVariable] This assignment to the local variable 'res' is
never read.
		res = target.pull().call();
		    ^
    (see https://errorprone.info/bugpattern/UnusedVariable)
```

Change-Id: I92e53dbe383fc16e9ba1a2fefbe2cf304bf0d8ad

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/PullCommandTest.java

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/PullCommandTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/PullCommandTest.java
index 5c33f03..2dc77cf 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/PullCommandTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/PullCommandTest.java
@@ -189,6 +189,9 @@ public void testPullConflictDiff3() throws Exception {
 				CONFIG_KEY_CONFLICTSTYLE, ConflictStyle.DIFF3);
 
 		res = target.pull().call();
+		assertFalse(res.getFetchResult().getTrackingRefUpdates().isEmpty());
+		assertTrue(res.getMergeResult().getMergeStatus()
+				.equals(MergeStatus.CONFLICTING));
 
 		String sourceChangeString = "Source change\n>>>>>>> branch 'master' of "
 				+ target.getRepository().getConfig().getString("remote",