When `g:skip_empty_sections` is set, `:AirlineRefresh` can be called
very often and cause slow down, because it forcefully re-creates the
highlighting groups. This is mostly not needed for the redraw to happen,
therefore, add the `<bang>` attribute to the command, making it skip to
re-create the highlighting groups and have all extensions that rely on a
forced update use the `!` form.
Should be much fast then.
fixes#1908
a753422549