Fix bug when viewing the commit diff page with non-ANSI files (#36149) (#36150)

Backport #36149

routers/web/repo/editor.go

@@ -312,11 +312,13 @@ func EditFile(ctx *context.Context) {
 				ctx.ServerError("ReadAll", err)
 				return
 			}
+			var fileContent string
 			if content, err := charset.ToUTF8(buf, charset.ConvertOpts{KeepBOM: true}); err != nil {
-				ctx.Data["FileContent"] = string(buf)
+				fileContent = string(buf)
 			} else {
-				ctx.Data["FileContent"] = content
+				fileContent = string(content)
 			}
+			ctx.Data["FileContent"] = fileContent
 		}
 	}