Write members of groups to NoteDb on group updates

The members of a group are stored in a separate file alongside the
one which is used for the basic group properties. Both files are
written to the same branch. In order to allow atomic updates of group
properties and members (e.g. during group creation), the implementation
in NoteDb uses only one commit to update both files.

The code is structured in a way that the implementation for subgroups
can be added easily in a follow-up change. We need to keep
GroupsUpdate#adNewGroupMembers for the moment because the creation of
a group is not prepared for NoteDb yet.

The added code regarding NoteDb won't be executed until we remove
the flag writeGroupsToNoteDb when the implementation for writing groups
to NoteDb is finished.

Change-Id: Ic61d4a088714e484dc3e45a522627ac662df55b7

java/com/google/gerrit/server/account/PutAgreement.java

@@ -39,6 +39,7 @@ import com.google.inject.Inject;
 import com.google.inject.Provider;
 import com.google.inject.Singleton;
 import java.io.IOException;
+import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.lib.Config;
 
 @Singleton
@@ -65,7 +66,7 @@ public class PutAgreement implements RestModifyView<AccountResource, AgreementIn
 
   @Override
   public Response<String> apply(AccountResource resource, AgreementInput input)
-      throws IOException, OrmException, RestApiException {
+      throws IOException, OrmException, RestApiException, ConfigInvalidException {
     if (!agreementsEnabled) {
       throw new MethodNotAllowedException("contributor agreements disabled");
     }