Ignore URLs starting with // (!5677)

CHANGELOG

@@ -8,6 +8,7 @@ v 8.11.0 (unreleased)
   - Convert switch icon into icon font (ClemMakesApps)
   - Remove magic comments (`# encoding: UTF-8`) from Ruby files. !5456 (winniehell)
   - Add support for relative links starting with ./ or / to RelativeLinkFilter (winniehell)
+  - Ignore URLs starting with // in Markdown links !5677 (winniehell)
   - Fix CI status icon link underline (ClemMakesApps)
   - The Repository class is now instrumented
   - Cache the commit author in RequestStore to avoid extra lookups in PostReceive

lib/banzai/filter/relative_link_filter.rb

@@ -35,6 +35,7 @@ module Banzai
 
       def process_link_attr(html_attr)
         return if html_attr.blank?
+        return if html_attr.value.start_with?('//')
 
         uri = URI(html_attr.value)
         if uri.relative? && uri.path.present?
@@ -92,7 +93,7 @@ module Banzai
         parts = request_path.split('/')
         parts.pop if uri_type(request_path) != :tree
 
-        path.sub!(%r{^\./}, '')
+        path.sub!(%r{\A\./}, '')
 
         while path.start_with?('../')
           parts.pop