refactor: clean up fragile diff render templates, use backend typed structs (#38517)

Blob.Size requires a context after the repository context removal
(5b078f72aa).

Actually the template code should just render, it should not depend on
the fragile dynamic calls to backend functions.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/diff/csv_diff.tmpl

@@ -1,6 +1,8 @@
+{{$root := ctx.RootData}}
+{{$file := $.file}}
 <tr>
 	<td>
-		{{$result := call .root.CreateCsvDiff .file .blobBase .blobHead}}
+		{{$result := call $root.CreateCsvDiff $file $file.LeftBlob $file.RightBlob}}
 		{{if $result.Error}}
 			<div class="ui center">{{$result.Error}}</div>
 		{{else if $result.Sections}}