That’s the key, no wonder the below Go template snippet feels natural to me:
{{ $.Scratch.Set "__repo_updated_time" (index (index (index (index $repo_json "commit") "commit") "committer") "date") }}
… I see it just like the below in Emacs-Lisp 
(setq foo (index (index (index (index repo-json "commit") "commit") "committer") "date"))