Fix error 500 when viewing commit and merge request diffs

Due to the refactoring in !16082, `Blob#batch` no longer falls back
to a default `blob_size_limit`. Since `Repository#batch_blobs` was using
a default `nil` value, this would cause issues in the `Blob#find_by_rugged`
method.

This fix here is to be consistent and use a non-nil default value in
`Repository#batch_blobs`.

The problem was masked in development and tests because Gitaly is always
enabled by default for all features.

Closes #41735

lib/gitlab/git/repository.rb

@@ -1187,7 +1187,7 @@ module Gitlab
       end
       # Items should be of format [[commit_id, path], [commit_id1, path1]]
-      def batch_blobs(items, blob_size_limit: nil)
+      def batch_blobs(items, blob_size_limit: Gitlab::Git::Blob::MAX_DATA_DISPLAY_SIZE)
         Gitlab::Git::Blob.batch(self, items, blob_size_limit: blob_size_limit)
       end

spec/lib/gitlab/diff/file_collection/merge_request_diff_spec.rb

@@ -19,4 +19,18 @@ describe Gitlab::Diff::FileCollection::MergeRequestDiff do
     diff_files
   end
+
+  shared_examples 'initializes a DiffCollection' do
+    it 'returns a valid instance of a DiffCollection' do
+      expect(diff_files).to be_a(Gitlab::Git::DiffCollection)
+    end
+  end
+
+  context 'with Gitaly disabled', :disable_gitaly do
+    it_behaves_like 'initializes a DiffCollection'
+  end
+
+  context 'with Gitaly enabled' do
+    it_behaves_like 'initializes a DiffCollection'
+  end
 end