Check validity of added feature flag files only

Signed-off-by: Rémy Coutable <remy@rymai.me>

danger/feature_flag/Dangerfile

@@ -34,12 +34,22 @@ module Tooling
       @helper = helper
     end
 
-    def check_touched_feature_flag_files
-      touched_feature_flag_files.each do |feature_flag|
+    def check_added_feature_flag_files
+      added_files.each do |feature_flag|
         check_feature_flag_yaml(feature_flag)
       end
     end
 
+    def check_modified_feature_flag_files
+      modified_files.each do |feature_flag|
+        check_default_enabled(feature_flag)
+      end
+    end
+
+    def feature_flag_file_added?
+      added_files.any?
+    end
+
     def feature_flag_file_touched?
       touched_feature_flag_files.any?
     end
@@ -201,9 +211,10 @@ feature_flag_dangerfile = Tooling::FeatureFlagDangerfile.new(
   helper: helper
 )
 
-feature_flag_dangerfile.check_touched_feature_flag_files
+feature_flag_dangerfile.check_added_feature_flag_files
+feature_flag_dangerfile.check_modified_feature_flag_files
 
-if helper.security_mr? && feature_flag_dangerfile.feature_flag_file_touched?
+if helper.security_mr? && feature_flag_dangerfile.feature_flag_file_added?
   failure("Feature flags are discouraged from security merge requests. Read the [security documentation](https://gitlab.com/gitlab-org/release/docs/-/blob/master/general/security/utilities/feature_flags.md) for details.")
 end