Compare with the most recently used window by default.

* lisp/vc/compare-w.el (compare-windows-get-window-function):
New defcustom.
(compare-windows-get-recent-window)
(compare-windows-get-next-window): New functions.
(compare-windows, compare-windows-sync-default-function):
Use `compare-windows-get-window-function' instead of `next-window'.
(compare-windows): Add diff/match messages with region boundaries.
Fixes: debbugs:19170