git-svn.txt: fix usage of --add-author-from The option '--add-author-from' is used in 'commit-diff', 'set-tree', and 'dcommit' subcommands. Signed-off-by: Valentin Haenel <valentin.haenel@gmx.de> Acked-by: Eric Wong <normalperson@yhbt.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>