Add initialisation of global autoload variables as a safeguard

This might not be necessary as these variables are at the moment properly
initialized when the plugin is loaded in the normal way.  But as they are
autoload variables vim will load the corresponding script *every* time they
are encounter uninitialized for some reason (which should never happen).
With these initialisation vim will load the file autoload/vundle.vim if one of
them is used but not set and then all of them will be set.
change g:vundle#changelog into s:changelog