[bug] C-u org-update-statitics-cookies errors out with "Non-existent agenda file" if current file isn't saved

[mail]

* Reproduction

1. emacs -Q
2. C-x C-f /tmp/new.org RET
3. Type anything
4. Type C-u C-c # (C-u org-update-statistics-cookies) to try to update all 
statistics cookies in the buffer
5. Notice the "Non-existent agenda file /tmp/new.org." error.

This doesn't happen if the file already exists.

I first saw this reported in this reddit thread: 
https://www.reddit.com/r/orgmode/comments/n9g1yf/is_this_a_bug_with_orgupdatestatisticcookies/
 (u/flyin1501 is me)

* Expectation

All statistics cookies in the buffer are updated no matter if the buffer's 
associated file is already created or not.

* Why this is happening

- `org-update-statistics-cookies` (C-c #) uses `org-map-entries` to update all 
cookies.
- `org-map-entries` uses `org-agenda-prepare-buffers` to prepare the file when 
SCOPE is nil. Excerpt:

        (if (not scope)
            (progn
              (org-agenda-prepare-buffers
               (and buffer-file-name (list buffer-file-name)))
              (setq res
                    (org-scan-tags
                     func matcher org--matcher-tags-todo-only start-level)))

- This function complains if the file does not exist, causing the error.

* Solution (IMO)

`org-map-entries` is quite complex because of its matching and scoping 
features; `org-update-statistics-cookies` only updates the current buffer and 
should use `org-map-region` instead. IMO it should go from

    (if all
        (progn
          (org-update-checkbox-count 'all)
          (org-map-entries 'org-update-parent-todo-statistics))

to:

    (if all
        (progn
          (org-update-checkbox-count 'all)
          (org-map-region 'org-update-parent-todo-statistics (point-min) 
(point-max)))