Trigger focus event after reload (#4931)

- Indexes restart after reload, so same-index item was not seen as new
- With --id-nth, compare by key. No event if focused item keeps its key.
- Skip focus check while track is blocked
- Document that binding reload to focus can loop

Close #4930

man/man1/fzf.1

@@ -1909,7 +1909,10 @@ e.g.
 \fIfocus\fR
 .RS
 Triggered when the focus changes due to a vertical cursor movement or a search
-result update.
+result update. After a reload, the focused item is considered new unless
+\fB\-\-id\-nth\fR is set and the item has the same key as the previously
+focused item. Because of this, binding \fBreload\fR to this event can cause an
+infinite loop.
 
 e.g.
      \fBfzf \-\-bind 'focus:transform\-preview\-label:echo [ {} ]' \-\-preview 'cat {}'