Add support for subversion version 3 delta dumps

[mocleiri]

The current converter uses subversion 2 dump streams where the entire file contents is emitted at each change which is wasteful for space.

Subversion version 3 dump streams use the svndiff format for the file contents which applies the provided data to parts of the original file (denoted from the copyfrom version of the file).

It would be very useful for the converter to be able to read the svndiff binary format to restore files at each revision directly.