Fix a memory leak caused by Banzai::Filter::SanitizationFilter

In Banzai::Filter::SanitizationFilter#customize_whitelist, we append
three lambdas that has reference to the SanitizationFilter instance,
which in turn (potentially) has a reference to the following chain:

context hash -> Project instance -> Repository instance -> lookup hash
-> various Rugged instances -> various mmap-ed git pack files.

All of the above is not garbage collected because the array we append
the lambdas to is the constant
HTML::Pipeline::SanitizationFilter::WHITELIST.

CHANGELOG

@@ -108,6 +108,7 @@ v 8.11.0 (unreleased)
   - Sort folders with submodules in Files view !5521
   - Each `File::exists?` replaced to `File::exist?` because of deprecate since ruby version 2.2.0
   - Add auto-completition in pipeline (Katarzyna Kobierska Ula Budziszewska)
+  - Fix a memory leak caused by Banzai::Filter::SanitizationFilter
 v 8.10.5
   - Add a data migration to fix some missing timestamps in the members table. !5670

lib/banzai/filter/sanitization_filter.rb

@@ -7,7 +7,7 @@ module Banzai
       UNSAFE_PROTOCOLS = %w(data javascript vbscript).freeze
       def whitelist
-        whitelist = super
+        whitelist = super.dup
         customize_whitelist(whitelist)
@@ -42,6 +42,8 @@ module Banzai
         # Allow any protocol in `a` elements...
         whitelist[:protocols].delete('a')
+        whitelist[:transformers] = whitelist[:transformers].dup
+
         # ...but then remove links with unsafe protocols
         whitelist[:transformers].push(remove_unsafe_links)