Use diff faces for compare-windows

* lisp/vc/compare-w.el: Require diff-mode for diff faces.
(compare-windows-removed, compare-windows-added): New faces
inheriting from diff faces.
(compare-windows): Define obsolete face alias.
(compare-windows-highlight): Replace face `compare-windows' with
new faces `compare-windows-added' and `compare-windows-removed'.
(compare-windows-get-recent-window): Signal an error when
no other window is found.

Fixes: debbugs:19451
Avoid compilation warning in saveplace.el for dired-current-directory