bug#34975: 26.1.92; In backquote, the comma marker does not work with variable name started with dot

[Xu Chunyang]

For example,

(let ((.foo 42))
  (list .foo))
;; => (42)

(let ((.foo 42))
  `(,.foo))
;; => ((\,\. foo))

I expect both of them returns (42). is it a bug?

The bug report is inspired by 
https://www.reddit.com/r/emacs/comments/b325ae/letplist_a_version_of_the_useful_letalist_macro/