vcs-svn: Skeleton of an svn delta parser

vcs-svn: Skeleton of an svn delta parser

A delta in the subversion delta (svndiff0) format consists of the
magic bytes SVN\0 followed by a sequence of windows, each beginning
with a window header consisting of five integers (with variable-length
representation):

	source view offset
	source view length
	output length
	instructions length
	auxiliary data length

Add an svndiff0_apply() function and test-svn-fe -d commandline tool
to parse such a delta in the special case of not including any
instructions or auxiliary data.

Later patches will add features to turn this into a fully functional
delta applier, for use by svn-fe in parsing the streams produced by
"svnrdump dump" and "svnadmin dump --deltas".

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