Refactor tmpl and blob_excerpt (#32967)

1. do not use `{{/* */}}` to remove spaces, use `{{- -}}`
2. fix "blob_excerpt" endpoint, remove the legacy fragile code: have
tested commit diff and wiki diff

templates/base/head_script.tmpl

@@ -1,5 +1,4 @@
-{{/*
-==== DO NOT EDIT ====
+{{/* ==== DO NOT EDIT ====
 If you are customizing Gitea, please do not change this file.
 If you introduce mistakes in it, Gitea JavaScript code wouldn't run correctly.
 */}}