[Bug: 21.4.22] and [Bug: 21.5-b28] ediff-coding-system-for-write 'emacs-internal

[Stephen J. Turnbull]

Jacques Duthen writes:

 > Debugger entered--Lisp error: (invalid-argument "No such coding
 > system" emacs-internal)

This is a known bug in upstream ediff, and I believe a solution has
been agreed on.  It may already be in the prerelease package
distribution.

In the meantime, you can workaround for the purposes of ediff with

(setq ediff-coding-system-for-write 'escape-quoted)