counsel.el (counsel-yank-pop): Truncate during display
* counsel.el (counsel-yank-pop-truncate): Remove defcustom.
(counsel-yank-pop-truncate-radius): New defcustom.
(counsel--yank-pop-truncate): New defun.
(counsel--yank-pop-format-function): New defun.
During the completion, only the context around the match will be shown.
By default, the context is +2 lines above and +2 lines below the match.
It can be adjusted with `counsel-yank-pop-truncate-radius'.
Additionally, `ivy-height' is temporarily bound to 5 during completion.
This way, the maximum minibuffer height should be 1+4*5=21 lines.