vcs-svn: More dump format sanity checks

vcs-svn: More dump format sanity checks

Node-action: change is not appropriate when switching between file and
directory or adding a new file.  Current svn-fe silently accepts such
nodes and the resulting tree has missing files in the "changed when
meant to add" case.

Node-action: add requires some content (text or directory); there is
no such thing as an "intent to add" node in svn dumps.  Current svn-fe
accepts such contentless adds but produces an invalid fast-import
stream that refers to nonexistent mark :0 in response.

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