Add ability to deny uploading merge commits

Teaches gerrit to deny merge commits when using the default
+2 read access level. To allow merge commits to be uploaded,
permission must be explicitly granted via +3 read access.

Change-Id: Idb0625aebbe4b554fa25e81aedcce36e66b58d6d
Signed-off-by: Jay Soffian <jaysoffian@gmail.com>

gerrit-server/src/test/java/com/google/gerrit/server/schema/SchemaCreatorTest.java

@@ -205,7 +205,7 @@ public class SchemaCreatorTest extends TestCase {
     } finally {
       c.close();
     }
-    assertValueRange(ApprovalCategory.READ, -1, 1, 2);
+    assertValueRange(ApprovalCategory.READ, -1, 1, 2, 3);
   }
 
   public void testCreateSchema_ApprovalCategory_Submit() throws OrmException {