Optimize the handling of binary items

When writing large binary items (e.g. audio or video files), creating a
hard links pointing to the original file makes the build a lot faster,
and does not duplicate the data on disk.

Hard linking will fail when the source file is not on the same device as
the output directory (e.g. when /tmp is a tmpfs). In that case, fallback
to the previous behavior which is slower, but should always work.

nanoc/lib/nanoc/base/services/item_rep_writer.rb

@@ -48,7 +48,13 @@ def write_single(item_rep, snapshot_repo, snapshot_name, raw_path, written_paths
       is_modified = is_created || !FileUtils.identical?(raw_path, temp_path)
 
       # Write
-      FileUtils.cp(temp_path, raw_path) if is_modified
+      if is_modified
+        begin
+          FileUtils.ln(temp_path, raw_path, force: true)
+        rescue Errno::EXDEV
+          FileUtils.cp(temp_path, raw_path)
+        end
+      end
 
       item_rep.modified = is_modified