replacing: keep centering the occurrence, also after toggling help lines

Normally, returning to the main loop will set 'focusing' back to TRUE,
but the replacement loop doesn't return until replacing is finished.

This fixes https://savannah.gnu.org/bugs/?61397.

Bug existed since version 5.0, commit d8249917.

src/prompt.c

@@ -771,6 +771,7 @@ int do_yesno_prompt(bool all, const char *msg)
 			titlebar(NULL);
 			focusing = FALSE;
 			edit_refresh();
+			focusing = TRUE;
 		}
 #endif
 		else