Eager-load image blob data in diffs

Since gitlab_git 8.0, blob data are lazy-loaded so we have to call
blob.load_all_data!(repo) to eager-load them.

Fixes #13458.

app/views/projects/diffs/_image.html.haml

 - diff = diff_file.diff
+- file.load_all_data!(@project.repository)
 - if diff.renamed_file || diff.new_file || diff.deleted_file
   .image
     %span.wrap
@@ -6,6 +7,7 @@
         %img{src: "data:#{file.mime_type};base64,#{Base64.encode64(file.data)}"}
       %p.image-info= "#{number_to_human_size file.size}"
 - else
+  - old_file.load_all_data!(@project.repository)
   .image
     %div.two-up.view
       %span.wrap