fix: fish prompt in root directory

files/fish/functions/fish_prompt.fish

@@ -18,8 +18,16 @@ end
 # Current working directory prompt
 function fish_cwd_prompt
     set -l cwd (pwd)
-    test $cwd = $HOME && printf "~" || \
+    switch "$cwd" 
-        printf "%s/%s" (basename (dirname $cwd)) (basename $cwd)
+        case "$HOME"
+            printf "~"
+        case "/"
+            printf "/"
+        case "*"
+           set -l parent_dir (basename (dirname $cwd))
+           test $parent_dir = "/" || printf "%s" $parent_dir
+           printf '/%s' (basename $cwd)
+    end
 end
 
 # Guix environment prompt