`findfunc` is called every wildtrigger()

```help
						*'findfunc'* *'ffu'* *E1514*
'findfunc' 'ffu'	string	(default empty)
...
	The function is called only once per |:find| command invocation.
	The function can process all the directories specified in 'path'.
...
```

The current (v9.2.957) behaviour is not align with docuemnts.

patch `9.1.810` has the correct behaviour.

CC: @yegappan 
