This is a step in the direction of making ediff behave better by
default under tiling window managers and fixing Bug#62164.
* lisp/vc/ediff-wind.el (ediff-floating-control-frame): Add
defcustom.
(ediff-frame-make-utility, ediff-setup-control-frame): Allow setting
the control frame up as a utility window under X. (Bug#62164)