forge--msg: Make it more resilient to buffer kills

[LemonBreezes]

Previously, if you killed the magit-status buffer while forge was
fetching the repository information, and ran magit-status again, the
forge status would be missing from the modeline. With this PR, we use
the buffer name as a backup to identify the status buffer in case it was
killed.

The reason why I want this change is that by default, Doom Emacs kills the
magit-status buffer when you quit magit "if the buried/killed magit buffer was
the last magit buffer open for this repo" (see https://github.com/doomemacs/doomemacs/blob/master/modules/tools/magit/autoload.el#L125).

[tarsius]

Closes #633.

[LemonBreezes]

Force-pushed because I realized I did not use buffer-live-p for buf.

[tarsius]

The Fetching page N status updates are not put in place by this function, but by ghub--graphql-set-mode-line. That function does not have any concept or access to "that repository" (except maybe deeply nested inside an url-encoded tree.

Doing this properly would be much more involved and while it would be desirable, I still do not think it is work the effort. FWIW before I realized the above, I cam up with the following. I cannot test it, obviously, and it still is an idealized version that doesn't take enough circumstances into account, but I guess it would be an improvement, if it weren't for the above.

@ -506,6 +506,16 @@ (defun forge--msg (repo echo done format &rest args)
                  msg)))
     (when (and echo msg)
       (message "%s%s" msg (if done "...done" "...")))
+    (cond
+     ((or (not format)
+          (not (and (derived-mode-p 'magit-mode)
+                    (equal default-directory (oref repo worktree))))))
+     ((not (buffer-live-p forge--mode-line-buffer))
+      (setq forge--mode-line-buffer (current-buffer)))
+     ((not (eq (current-buffer) forge--mode-line-buffer))
+      (with-current-buffer forge--mode-line-buffer
+        (setq mode-line-process nil))
+      (setq forge--mode-line-buffer (current-buffer))))
     (when (buffer-live-p forge--mode-line-buffer)
       (with-current-buffer forge--mode-line-buffer
         (setq mode-line-process

Instead I have added a new variable ghub-graphql-message-progress. If you enable that, then the Fetching page N messages are shown in the echo area.