gptel-org: Don't include tags in gptel-org-set-topic (#801)

gptel-org.el (gptel-org-set-topic): The function `org-get-heading`
returns the heading text along with any tags if the heading has
tags. In contrast, the `ITEM` property gives the actual heading
text of the entry and should be used for that purpose. When
available, the function uses the org-element cache to improve
efficiency.

Author: akirak

gptel-org.el

@@ -184,7 +184,7 @@ current heading and the cursor position."
                                  (substring-no-properties
                                   (replace-regexp-in-string
                                    "\\s-+" "-"
-                                   (org-get-heading)))
+                                   (org-entry-get nil "ITEM")))
                                  50))))))
   (when (stringp topic) (org-set-property "GPTEL_TOPIC" topic)))