fish_print_hg_root: Don't break if $PWD includes newlines

Fixes #11348

share/functions/fish_print_hg_root.fish

@@ -7,16 +7,16 @@ function fish_print_hg_root
     # Find an hg directory above $PWD
     # without calling `hg root` because that's too slow
     set -l root
-    set -l dir (pwd -P 2>/dev/null)
+    set -l dir "$(pwd -P 2>/dev/null)"
     or return 1
 
-    while test $dir != /
+    while not contains -- "$dir" "" / .
         if test -f $dir'/.hg/dirstate'
             echo $dir/.hg
             return 0
         end
         # Go up one directory
-        set dir (string replace -r '[^/]*/?$' '' $dir)
+        set dir (path dirname -- $dir)
     end
 
     return 1