forge--clone-buffer-topics-spec: New function

Beside cloning `forge--buffer-topics-spec', also ensure that that
itself is initialized.  Normally this is done when `magit-status-mode'
is enabled, but if that happens before this package is loaded, then it
is too late for the hook function to run, so we do it like this instead.

Re #681.

lisp/forge-issue.el

@@ -211,7 +211,7 @@ and optional HEADING to change the section heading."
   (when-let (((forge-db t))
              (repo (forge-get-repository :tracked?))
              ((oref repo issues-p))
-             (spec (if sspec spec (clone forge--buffer-topics-spec)))
+             (spec (if sspec spec (forge--clone-buffer-topics-spec)))
              ((memq (oref spec type) '(topic issue))))
     (oset spec type 'issue)
     (forge--insert-topics 'issues

lisp/forge-pullreq.el

@@ -264,7 +264,7 @@ Optional SPEC can be used to override that filtering specification,
 and optional HEADING to change the section heading."
   (when-let (((forge-db t))
              (repo (forge-get-repository :tracked?))
-             (spec (if sspec spec (clone forge--buffer-topics-spec)))
+             (spec (if sspec spec (forge--clone-buffer-topics-spec)))
              ((memq (oref spec type) '(topic pullreq))))
     (oset spec type 'pullreq)
     (forge--insert-topics 'pullreqs

lisp/forge-topic.el

@@ -339,6 +339,10 @@ an error."
           (clone forge-status-buffer-default-topic-filters))))
 (add-hook 'magit-status-mode-hook #'forge--init-buffer-topics-spec)
 
+(defun forge--clone-buffer-topics-spec ()
+  (forge--init-buffer-topics-spec)
+  (clone forge--buffer-topics-spec))
+
 (defclass forge--topics-spec ()
   ((type        :documentation "\
 Limit list based on topic type."