Finally, it worked, with the help from tmux-users mailing list, but there is still a problem with 1.8
# if if-shell goes in the beggining it will break the following command. this is going to be fixed after 1.8
# make c-a work as c-b
if-shell 'test -z "$SSH_CLIENT"' 'set -g prefix C-a'
https://github.com/brauliobo/gitconfig/blob/master/configs/.tmux.conf