Adapt ediff to nonstandard layouts

Make ediff cope with having some of its windows (especially the control
window) not shown by a custom ediff-window-setup-function.
Modernize relevant adjacent code. After this change, one can write a
custom ediff-window-setup-function that doesn't show the control window.

* doc/misc/ediff.texi (Notes on Heavy-duty Customization): Refine
language to explain that the window setup function doesn't have to show
all windows.
* lisp/vc/ediff-util.el (ediff-select-control-window-on-setup):
New variable.
(ediff-setup, ediff-recenter, ediff-recenter-one-window)
(ediff-recenter-ancestor, ediff-toggle-read-only)
(ediff-operate-on-windows, ediff-jump-to-difference-at-point)
(ediff-default-suspend-function)
(ediff-clone-buffer-for-region-comparison)
(ediff-clone-buffer-for-window-comparison): Modernize control flow;
select only windows that exist.
* lisp/vc/ediff-wind.el (ediff-with-live-window): New convenience macro.
(ediff-window-setup-function): Explain relaxed contract.

1 files changed, 4 insertions, 2 deletions

diff --git a/doc/misc/ediff.texi b/doc/misc/ediff.texi
index 26a0ee433d0..73f751f3a4b 100644
--- a/doc/misc/ediff.texi
+++ b/doc/misc/ediff.texi
@@ -2399,10 +2399,12 @@ The window displaying buffer A@.  If buffer A is not visible, this variable
 is @code{nil} or it may be a dead window.
 
 @item ediff-window-B
-The window displaying buffer B.
+The window displaying buffer B.  If buffer B is not visible, this variable
+is @code{nil} or it may be a dead window.
 
 @item ediff-window-C
-The window displaying buffer C, if any.
+The window displaying buffer C, if any.  If buffer C is not visible,
+this variable is @code{nil} or it may be a dead window.
 
 @item ediff-control-frame
 A dedicated frame displaying the control buffer, if it exists.  It is