(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 $(realpath " file ") 2> /dev/null"
           " | wc -l | tr -d '\n' | sed -e 's/^0$/-/'")))

;; (update-nb-git-commits-per-file-alist "/home/e/prjs/hf/project.clj")
(defun update-nb-git-commits-per-file-alist (file)
  ;; we both update the alist and return the nb of commits for file
  (cdr (car (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)
  (or (cdr (assoc file nb-git-commits-per-file-alist))
      (update-nb-git-commits-per-file-alist file)))