don't make the patch buffer RO.

don't make the patch buffer RO.

(ediff-load-version-control): now checks if
ediff-revision-key is non-nil.

(ediff-revision-key): Now defaults to nil.

(ediff-revision): now takes prefix argument; can compare
two versions of the same file.

the merge buffer now assumes the major mode of the
default variant.

(ediff-documentation: new function.

(ediff-patch-buffer): now handles buffers that don't
visit any file.
(ediff-windows): renamed to ediff-windows-wordwise, added
(ediff-windows-linewise): new function.
Changed ediff-small/large-regions to ediff-regions-wordwise/linewise

Converted xemacs *screen* nomenclature to *frame*.
Incorporated overlay strings. Ediff no longer runs under emacs
19.28 and earlier and XEmacs 19.11 and earlier.

Changed window-system to ediff-window-display.