fish_config: Fully erase existing globals before replacing

We only erase existing globals for some of the theme-related variables
but not for all the `known_colors`, causing `fish_config` to still emit
warnings for these if saving a theme choice after trying it.

share/functions/fish_config.fish

@@ -267,6 +267,9 @@ function fish_config --description "Launch fish's web based configuration"
                         contains -- $c $have_colors
                         and continue
 
+                        # Erase conflicting global variables so we don't get a warning and
+                        # so changes are observed immediately.
+                        set -eg $c
                         set $scope $c
                     end