vcs-svn: Error out for v3 dumps

vcs-svn: Error out for v3 dumps

By ignoring the Text-Delta and Prop-Delta node fields, current svn-fe
happily mistakes deltas for full text and instead of cleanly erroring
out, it produces a valid but semantically bogus fast-import stream
when fed a dump file in the modern "svnadmin dump --deltas" format.

Dump file parsers are supposed to ignore header fields they don't
understand (to allow for backward-compatible extensions), but they are
also supposed to check the SVN-fs-dump-format-version header to
prevent misinterpretation of non backward-compatible extensions.
Do so.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>