bug#21644: 24.4; completing-read acts differently on functional collection

[Oleh Krehel]

> Try typing "g" followed by RET for both of the above. In the first
> case (collection is a list), it returns "green". In the second case
> (collection is a function), it returns just "g". This is reproducible
> in emacs -Q.

I can reproduce this as well, although "TAB RET" works where "RET"
doesn't. I attach a patch with a fix. If someone can check it I can push
it. The patch doesn't solve all the intricacies of
`completion-ignore-case' that were mentioned in that function's
comments, just this specific use case.

    Oleh

0001-lisp-minibuffer.el-completion-complete-and-exit-Fix-.patch
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