fish_config theme show: list default scheme first

Webconfig does the same ("Add the current scheme first, then the
default.").

share/functions/__fish_theme_for_each.fish

@@ -3,7 +3,15 @@
 function __fish_theme_for_each
     set -l callback $argv[1]
     set -l theme_names $argv[2..]
-    for theme_name in (__fish_theme_names $theme_names)
+    for theme_name in (
+        set -l matching_names (__fish_theme_names $theme_names)
+        if not set -q theme_names[1]
+            echo default
+            string match -v -- default $matching_names
+        else
+            string join -- \n $matching_names
+        end
+    )
         set -l theme_data (__fish_theme_cat $theme_name)
         $callback \
             --name=$theme_name \

tests/checks/fish_config.fish

@@ -149,11 +149,11 @@ fish_config theme show ayu-dark ayu-light | string match -r '^.*ayu.*'
 mkdir $__fish_config_dir/themes
 touch $__fish_config_dir/themes/custom-from-userconf.theme
 fish_config theme show | grep -E '[^-]default|base16-default-dark|custom-from-userconf'
+# CHECK: {{.*}}default{{\x1b\[m}}
 # CHECK: {{.*}}custom-from-userconf{{\x1b\[m}}
 # CHECK: {{.*}}base16-default-dark{{\x1b\[m}}
 # CHECK: {{.*}}default-dark{{\x1b\[m}}
 # CHECK: {{.*}}default-light{{\x1b\[m}}
-# CHECK: {{.*}}default{{\x1b\[m}}
 
 # Override a default theme with different colors.
 __fish_data_with_file themes/none.theme \