* lisp/vc/ediff-wind.el: Don't pop spurious frame in corner case

* lisp/vc/ediff-wind.el: Don't pop spurious frame in corner case

(ediff-setup-windows-multiframe-compare): Clarify control-flow.
Postpone the initial call to ediff-skip-unsuitable-frames until we're
sure it's needed to avoid creating a spurious new frame.
(ediff-setup-windows, ediff-setup-windows-plain)
(ediff-setup-windows-plain-merge, ediff-setup-control-frame)
(ediff-setup-windows-plain-compare, ediff-setup-windows-multiframe)
(ediff-setup-windows-multiframe-merge): Use with-current-buffer.