vcs-svn: Eliminate node_ctx.mark global

vcs-svn: Eliminate node_ctx.mark global

The mark variable is only used in handle_node().  Its life is
very short and simple: first, a new mark number is allocated if
this node has text attached, then that mark is recorded in the
in-core tree being built up, and lastly the mark is communicated
to fast-import in the stream along with the associated text.

A new reader may worry about interaction with other code, especially
since mark is not initialized to zero in handle_node() itself.
Disperse such worries by making it local.  No functional change
intended.

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