vcs-svn: Allow simple v3 dumps (no deltas yet)

vcs-svn: Allow simple v3 dumps (no deltas yet)

Since the dumpfile version 1 days, the Subversion dump format
gained some new fields:

 - a unique identifier for the repository (version 2 format)
 - whether the text and properties for a node should be
   interpreted as deltas
 - checksums for a delta's preimage
 - SHA-1 sums as alternatives to the existing MD5 checksums for
   copy source and the payload (delta).

For now what is relevant to us is the Text-delta and Prop-delta
fields, since not noticing these causes a dump file to be
misinterpreted (see the previous commit).

[jn: with tests]

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>