Generic Diff format

http://www.w3.org/TR/NOTE-gdiff-19970901

The output is a series of tokens:

   There are two kinds of GDIFF commands. The first kind is the DATA
   command (1 through 248). Each data command is followed by a number
   of data bytes which are copied onto the output stream.

   The second kind of GDIFF command is the COPY command (249 through
   255). Each COPY command is followed by two arguments: position and
   length. The arguments specify the portion of the old file that must
   be copied onto the output stream.

This doesn't tell us where to put the signature data, but if we're
going to use a standard differenc format it's probably better to keep
the signature data out of it.  Mm, perhaps it would be nice if the
protocol allowed for `application-defined' commands, but that's not so
bad.  

We can put the server-generated signature after the EOF token or in an
HTTP Trailer.  There's something quite symmetrical about putting it in
a trailer, since it's going to come back up to the server in an HTTP
Header. 

I wasn't so positive on this format in the past, but after talking to
paulus a bit it looks really good: