plugin autocomplete: update partial on manual trigger

data/plugins/autocomplete.lua

@@ -588,8 +588,11 @@ function autocomplete.open(on_close)
   end
 
   local av = get_active_view()
+  if av then
+    partial = get_partial_symbol()
     last_line, last_col = av.doc:get_selection()
     update_suggestions()
+  end
 end
 
 function autocomplete.close()