Use TestRepository's Git and Repository in acceptance tests

Avoid storing two references that have to be kept in sync, now that a
Git is available directly from TestRepository. Use a git() convenience
method to AbstractDaemonTest to shorten this. Similarly, add a repo()
convenience method to get the Repository.

Change-Id: I1ec8af8fead0876a2836f7972b5caf603847b25d

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/git/AbstractPushForReview.java

@@ -68,7 +68,7 @@ public abstract class AbstractPushForReview extends AbstractDaemonTest {
       default:
         throw new IllegalArgumentException("unexpected protocol: " + p);
     }
-    setRepo(cloneProject(project, url + "/" + project.get()));
+    testRepo = cloneProject(project, url + "/" + project.get());
   }
 
   @Test