GroupCheckerImpl: Mock directly the repository in unit-tests

The unit-tests of GroupCheckerImpl were targeting the internal
behaviour of how the repository works rather than checking directly
the expected outcome. That was working until the JGit internal
behaviour changes and the tests are failing without an apparent
reason.

Mock directly the Repository.exactRef() which is the method
that's expected to be called when the groups are checked.

Change-Id: I156c816803c3d3a87119f24e6839626a07034176

src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java
index 09f8f59..c612119 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/multisite/index/GroupCheckerImplTest.java
@@ -30,7 +30,6 @@
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectIdRef;
 import org.eclipse.jgit.lib.Ref;
-import org.eclipse.jgit.lib.RefDatabase;
 import org.eclipse.jgit.lib.Repository;
 import org.junit.Before;
 import org.junit.Test;
@@ -45,13 +44,11 @@
 
   GroupCheckerImpl objectUnderTest;
   @Mock private GitRepositoryManager repoManagerMock;
-  @Mock private RefDatabase refDatabaseMock;
   @Mock private Repository repoMock;
 
   @Before
   public void setUp() throws Exception {
     doReturn(repoMock).when(repoManagerMock).openRepository(allUsers);
-    doReturn(refDatabaseMock).when(repoMock).getRefDatabase();
     objectUnderTest = new GroupCheckerImpl(repoManagerMock, allUsers);
   }
 
@@ -118,6 +115,6 @@
       throws IOException {
     String groupRefName = RefNames.refsGroups(AccountGroup.uuid(groupUUID.toString()));
     ObjectIdRef.Unpeeled aRef = new ObjectIdRef.Unpeeled(Ref.Storage.LOOSE, groupRefName, objectId);
-    doReturn(objectId == null ? null : aRef).when(refDatabaseMock).exactRef(groupRefName);
+    doReturn(objectId == null ? null : aRef).when(repoMock).exactRef(groupRefName);
   }
 }