if the mark isn't on, allow Meta-} and Meta-{ to indent and unindent

if the mark isn't on, allow Meta-} and Meta-{ to indent and unindent
only the current line, just as it would if the mark covered only the
current line, instead of displaying a statusbar message and quitting

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3758 35c25a1d-7b9e-4130-9fde-d3aeb78583b8