Support lifting topic limit on the fly when using Helm

When using built-in completion, vertico, and I assume most other
completion frameworks, `forge-read-topic-lift-limit' can call some
function which causes the `completion-table-dynamic' function to be
called again.

Calling `helm-force-update' does not do that.  It also does not call
`forge--read-topic' again.  It does however do some recalculations
(I don't understand how), which do take `forge-limit-topic-choices'
into account.  When that calculation takes place, the minibuffer is
not current, and because we temporarily lift the limit by setting
`forge-limit-topic-choices' to nil in that buffer, that means that
for Helm the limit is still in place.

Trying to counter act this by making the minibuffer, or the helm
buffer, current when setting the variable and/or when calling
`helm-force-update', did not work.

So we have to set the global value instead.  But that would mean
that the filter remains off for all future invocations.  So we have
to re-enable filtering, every time `forge--read-topic' is invoked.
But some users might want to actually remove the filter for good,
so we have to give them another variable that the can use to make
their preference known: `forge-read-topic-lift-limit-for-helm'.

See emacs-helm/helm#2744.

Author: tarsius

lisp/forge-topic.el

@@ -46,7 +46,10 @@
 ;;; Options
 
 (defcustom forge-limit-topic-choices t
-  "Whether to initially limit completion candidates to active topics."
+  "Whether to initially limit completion candidates to active topics.
+
+For Helm users setting this option has no effect.  Instead they have
+to set the value of the variable `forge-limit-topic-choices-for-helm'."
   :package-version '(forge . "0.4.0")
   :group 'forge
   :type 'boolean)
@@ -719,7 +722,12 @@ can be selected from the start."
                      (forge--topics-spec :type 'topic :active t)
                      (forge--topics-spec :type 'topic :active nil :state nil)))
 
+(defvar forge-limit-topic-choices-for-helm t
+  "Like `forge-limit-topic-choices' (which see) but for Helm users.")
+
 (defun forge--read-topic (prompt current active all)
+  (when (bound-and-true-p helm-mode)
+    (setq forge-limit-topic-choices forge-limit-topic-choices-for-helm))
   (let* ((current (funcall current))
          (repo    (forge-get-repository (or current :tracked)))
          (default (and current (forge--format-topic-line current)))
@@ -772,16 +780,22 @@ can be selected from the start."
   (interactive)
   (when (and (minibufferp)
              forge-limit-topic-choices)
-    (setq-local forge-limit-topic-choices nil)
-    (forge-read-topic--remove-prompt-hint)
     (cond
       ((and (bound-and-true-p vertico-mode)
             (boundp 'vertico--input)
             (fboundp 'vertico--exhibit))
+       (setq-local forge-limit-topic-choices nil)
        (setq vertico--input t)
        (vertico--exhibit))
-      ((minibuffer-completion-help (minibuffer--completion-prompt-end)
-                                   (point-max))))))
+      ((and (bound-and-true-p helm-mode)
+            (fboundp 'helm-force-update))
+       (setq forge-limit-topic-choices nil)
+       (helm-force-update))
+      (t
+       (setq-local forge-limit-topic-choices nil)
+       (minibuffer-completion-help (minibuffer--completion-prompt-end)
+                                   (point-max))))
+    (forge-read-topic--remove-prompt-hint)))
 
 (defun forge-read-topic--remove-prompt-hint ()
   (when (minibufferp)