Prevent Git operations from checking replication lag on non-Geo sites

We were checking if the database is read only, and verifying the
replication lag, however with the addition of maintenance mode, the
read only check is also true when maintenance mode is enabled.

The problem with only using the read only check is two-fold:
unnecessary replication lag queries, when the current site is not
a secondary, as maintenance mode doesn't pause any DB operations;
and breaking databases that don't support the standard replication
functions to check the lag (i.e. Aurora).

Changelog: fixed
EE: true

ee/lib/ee/gitlab/geo_git_access.rb

@@ -55,7 +55,8 @@ module EE
       end
 
       def current_replication_lag_message
-        return if ::Gitlab::Database.read_write? || current_replication_lag == 0
+        return unless ::Gitlab::Geo.secondary?
+        return if current_replication_lag == 0
 
         "Current replication lag: #{current_replication_lag} seconds"
       end

ee/spec/lib/gitlab/git_access_spec.rb

@@ -331,6 +331,20 @@ RSpec.describe Gitlab::GitAccess do
     context 'git pull' do
       it { expect { pull_changes }.not_to raise_error }
 
+      context 'for non-Geo with maintenance mode' do
+        before do
+          stub_maintenance_mode_setting(true)
+        end
+
+        it 'does not return a replication lag message nor call the lag check' do
+          allow_next_instance_of(Gitlab::Geo::HealthCheck) do |instance|
+            expect(instance).not_to receive(:db_replication_lag_seconds)
+          end
+
+          expect(pull_changes.console_messages).to be_empty
+        end
+      end
+
       context 'for a secondary' do
         let(:current_replication_lag) { nil }