Load only blobs visible on the paginated page

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/363065

Changelog: fixed

app/controllers/projects/compare_controller.rb

@@ -126,7 +126,7 @@ def define_commits
   end
 
   def define_diffs
-    @diffs = compare.present? ? compare.diffs(diff_options) : []
+    @diffs = compare.present? ? compare.diffs(diff_options.merge(manual_load: true)) : []
   end
 
   def define_environment

app/helpers/commits_helper.rb

@@ -137,7 +137,7 @@ def commit_signature_badge_classes(additional_classes)
 
   def conditionally_paginate_diff_files(diffs, paginate:, page:, per:)
     if paginate
-      Kaminari.paginate_array(diffs.diff_files.to_a).page(page).per(per)
+      Kaminari.paginate_array(diffs.diff_files.to_a).page(page).per(per).tap { |diff_files| diff_files.each(&:lazy_load!) }
     else
       diffs.diff_files
     end

lib/gitlab/diff/file.rb

@@ -30,6 +30,7 @@ def initialize(
         diff_refs: nil,
         fallback_diff_refs: nil,
         stats: nil,
+        manual_load: nil,
         unique_identifier: nil)
 
         @diff = diff
@@ -41,6 +42,13 @@ def initialize(
         @unfolded = false
 
         # Ensure items are collected in the the batch
+        unless manual_load
+          new_blob_lazy
+          old_blob_lazy
+        end
+      end
+
+      def lazy_load!
         new_blob_lazy
         old_blob_lazy
       end

lib/gitlab/diff/file_collection/base.rb

@@ -125,6 +125,7 @@ def decorate_diff!(diff)
                                  repository: project.repository,
                                  diff_refs: diff_refs,
                                  fallback_diff_refs: fallback_diff_refs,
+                                 manual_load: diff_options.fetch(:manual_load, false),
                                  stats: stats)
 
           if @use_extra_viewer_as_main && diff_file.has_renderable?