Note that line highlighting requires signs to be placed (because the
line highlight is simply an attribute of a sign). If the user doesn't
want to see signs in the sign column, but does want line highlighting,
then we make the signs in the sign column invisible.
If neither the signs in the sign column nor the line highlights are
needed (presumably the user just looks at the hunks stats) then we can
remove all signs, at which point Vim removes the sign column...unless
the "sign column always" option is set.
