;; in elisp land associative arrays are usually lists, not maps...
(setq nb-git-commits-per-file-alist '(()))

;; (nb-git-commits-for-file "/tmp/nosuchfile")              ;;  ->  -
;; (nb-git-commits-for-file "~/path/to/file/in/a/git/repo") ;;  ->  nb commits
(defun nb-git-commits-for-file (file)
  ;; Very fugly shell command but it should get the job done
  (shell-command-to-string
   (concat "cd $(dirname $(realpath " file "))"
           " && git log --oneline $(realpath " file ") 2> /dev/null"
           " | wc -l | tr -d '\n' | sed -e 's/^0$/-/'")))

(defun update-nb-git-commits-per-file-alist (file)
  (add-to-list 'nb-git-commits-per-file-alist
               (cons file (nb-git-commits-for-file file))))

;; (find-cached-nb-git-commits-or-update-alist "/tmp/nosuchfile") ;;  ->  -
(defun find-cached-nb-git-commits-or-update-alist (file)
  (let* ((maybe-res (assoc file nb-git-commits-per-file-alist)))
    (if maybe-res
        (cdr maybe-res)
      (progn
        ;; we haven't cached the number of git commits for that file yet
        (update-nb-git-commits-per-file-alist file)
        ;; now the entry should be in the alist
        (cdr (assoc file nb-git-commits-per-file-alist))))))