Looks good to me!

If you want to print only one "[source]:" I suggest the following,
untested:

* `(car (magit-staged-files))` ($1): Modify package to use stable git references.
`(with-temp-buffer
   (magit-git-wash #'magit-diff-wash-diffs
     "diff" "--staged")
   (let ((source-block "[source]: ")
         (wrote-source-block? nil))
     (beginning-of-buffer)
     (when (search-forward "(method git-fetch" nil 'noerror)
       (setq wrote-source-block? t)
       (print (format "%sUse GIT-FETCH." source-block)))
     (beginning-of-buffer)
     (when (search-forward "(file-name (git-file-name" nil 'noerror)
       (print (format "%sUse GIT-FILE-NAME." (if wrote-source-block? "" "[source]:"))))))`


-- 
Pierre Neidhardt
https://ambrevar.xyz/