Paul Eggert [off-list ref] writes:

Here is the proposed format.  Each file name is a string of bytes, in
one of the following two formats:

A.  A nonempty sequence of ASCII graphic characters (i.e., bytes in
    the range '!' == '\041' through '~' == '\177').  The first byte
    cannot be '!' == '\041' or '"' == '\042'.  Leading '"' is used for
    (B) below, and leading '!' is reserved for future extensions.

B.  A nonempty C-language character string literal, with the following
    restrictions and modifications:

    B1.  No multibyte character processing is done.  Members of the
         string literal are treated as bytes, not characters.  Null
         bytes are not allowed, and '"' == '\042', '\\' == '\134' and
         '\n' == '\012' are allowed only if properly escaped as shown
         below; but all other bytes are allowed.

    B2.  No trigraph processing is done (e.g., ??/ stands for three
         bytes, not one).

    B3.  No line-splicing is done (i.e., backslash-newline is not allowed).

    B4.  Only the following escape sequences are allowed.

           \" \\ \a \b \f \n \r \t \v
           \XYZ  (where X, Y, and Z are octal digits, X <= 3, and
                  at least one of the digits is nonzero)

Just to let you know, I am slowly converting apply.c to accept
this format, and also diff.c to produce this.  I did not
personally like the missing double quotes around what I did
anyway, although it was easier to code.