Make BranchResource and TagResource extend a common class

Add a new abstract class, RefResource, that defines access methods to
get the ref name and revision.

Make both BranchResource and TagResource derive from RefResource, and
add the missing implementations in TagResource.

This will make it easier to consolidate common code that can be used
for both branch and tag related endpoints.

Change-Id: Ib44d621e34ce2b1077349db0037d960155b790e0

gerrit-server/src/main/java/com/google/gerrit/server/project/BranchResource.java

@@ -19,7 +19,7 @@ import com.google.gerrit.extensions.restapi.RestView;
 import com.google.gerrit.reviewdb.client.Branch;
 import com.google.inject.TypeLiteral;
 
-public class BranchResource extends ProjectResource {
+public class BranchResource extends RefResource {
   public static final TypeLiteral<RestView<BranchResource>> BRANCH_KIND =
       new TypeLiteral<RestView<BranchResource>>() {};
 
@@ -38,10 +38,12 @@ public class BranchResource extends ProjectResource {
     return new Branch.NameKey(getNameKey(), branchInfo.ref);
   }
 
+  @Override
   public String getRef() {
     return branchInfo.ref;
   }
 
+  @Override
   public String getRevision() {
     return branchInfo.revision;
   }