vcs-svn: Use mark to indicate nodes with included text

vcs-svn: Use mark to indicate nodes with included text

Allocate a mark if needed as soon as possible so later code can use
"if (mark)" to check if this node has text attached rather than
explicitly checking for Text-content-length.

While at it, reject directory nodes with text attached; the presence
of such a node would indicate a bug in the dump generator or svn-fe's
understanding.  In the long term, it would be nice to be able to
continue parsing and save the error for later, but for now it is
simpler to error out right away.

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