bug#60411: 29.0.60; minibuffer-next-completion skips first candidate when completions-header-format and completion-show-help are nil

[Gregory Heytings]

> Yuck.  I get the impression that it would be cleaner to fix "the first 
> call to `minibuffer-next-completion`" instead.

That was my impression, too.  Until I tried to fix "the first call to 
`minibuffer-next-completion`".  But I may have missed something, of 
course.