Insert minibuffer history wholesale

* counsel.el (counsel-minibuffer-history): Replace minibuffer
contents with selected history element, rather than just appending.

Fixes abo-abo#1484.

counsel.el

@@ -4831,6 +4831,7 @@ An extra action allows to switch to the process buffer."
     (ivy-read "History: " (ivy-history-contents minibuffer-history-variable)
               :keymap ivy-reverse-i-search-map
               :action (lambda (x)
+                        (delete-minibuffer-contents)
                         (insert (substring-no-properties (car x))))
               :caller 'counsel-minibuffer-history)))