Add button to vc-dir to toggle visibility of stash list

Add button to vc-dir to toggle visibility of stash list

* lisp/vc/vc-git.el: Move cl-lib require outside 'eval-when-compile'.

* lisp/vc/vc-git.el (vc-git-show-stash):New user option.
(vc-git-make-stash-button): Create button that allows hiding the stash
list.
(vc-git-dir-extra-headers): Split stash list into hideable and
non-hideable parts depending on value of vc-git-show-stash.  Add
button to toggle visibility of hideable part.

* etc/NEWS: Announce it.