vcs-svn: let deltas use data from preimage

vcs-svn: let deltas use data from preimage

The copyfrom_source instruction appends data from the preimage buffer
to the end of output.  Its arguments are a length and an offset
relative to the beginning of the source view.

With this change, the delta applier is able to reproduce all 5,636,613
blobs in the early history of the ASF repository.  Tested with

	mkfifo backflow
	svn-fe <svn-asf-public-r0:940166 3<backflow |
	git fast-import --cat-blob-fd=3 3>backflow

with svn-asf-public-r0:940166 produced by whatever version of
Subversion the dumps in /dump/ on svn.apache.org use (presumably
1.6.something).

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