Unconditionally setup topics spec in status buffers

A user may set `forge-add-default-sections' to nil in and then
manually add these sections in a different place.  Previously they
would have had to also manually add `forge--init-buffer-topics-spec'
to `magit-status-mode-hook', but that's an internal detail, they
should not have to know about.

Closes #681.

lisp/forge-topic.el

@@ -333,6 +333,12 @@ an error."
 (defvar-local forge--buffer-topics-spec nil)
 (put 'forge--buffer-topics-spec 'permanent-local t)
 
+(defun forge--init-buffer-topics-spec ()
+  (unless forge--buffer-topics-spec
+    (setq forge--buffer-topics-spec
+          (clone forge-status-buffer-default-topic-filters))))
+(add-hook 'magit-status-mode-hook #'forge--init-buffer-topics-spec)
+
 (defclass forge--topics-spec ()
   ((type        :documentation "\
 Limit list based on topic type."

lisp/forge.el

@@ -68,15 +68,9 @@
 If you want to disable this, then you must set this to nil before
 `forge' is loaded.")
 
-(defun forge--init-buffer-topics-spec ()
-  (unless forge--buffer-topics-spec
-    (setq forge--buffer-topics-spec
-          (clone forge-status-buffer-default-topic-filters))))
-
 (when forge-add-default-sections
   (magit-add-section-hook 'magit-status-sections-hook #'forge-insert-pullreqs nil t)
-  (magit-add-section-hook 'magit-status-sections-hook #'forge-insert-issues   nil t)
-  (add-hook 'magit-status-mode-hook #'forge--init-buffer-topics-spec))
+  (magit-add-section-hook 'magit-status-sections-hook #'forge-insert-issues   nil t))
 
 ;;; Add Bindings