vcs-svn: let deltas use data from postimage

vcs-svn: let deltas use data from postimage

The copyfrom_target instruction copies appends data that is already
present in the current output view to the end of output.  (The offset
argument is relative to the beginning of output produced in the
current window.)

The region copied is allowed to run past the end of the existing
output.  To support that case, copy one character at a time rather
than calling memcpy or memmove.  This allows copyfrom_target to be
used once to repeat a string many times.  For example:

	COPYFROM_DATA 2
	COPYFROM_OUTPUT 10, 0
	DATA "ab"

would produce the output "ababababababababababab".

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Ramkumar Ramachandra <artagnon@gmail.com>