vcs-svn: implement copyfrom_data delta instruction

The copyfrom_data instruction copies a few bytes verbatim from the
novel text section of a window to the postimage.

[jn: with memory leak fix from David]

Improved-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Ramkumar Ramachandra <artagnon@gmail.com>