__fish_data_list_files: fix for installed builds

This is only used in test code where bad output is ignored.
To be tested by a following commit.

share/functions/__fish_data_list_files.fish

@@ -9,7 +9,7 @@ function __fish_data_list_files
             set dir $__fish_data_dir
         end
         set -l files $dir/**
-        string replace -- $dir/ '' $files
+        string replace -- $__fish_data_dir/ '' $files
     else
         status list-files $dir
     end

share/functions/__fish_data_with_file.fish

@@ -3,7 +3,11 @@ function __fish_data_with_file
     set -l path $argv[1]
     set -l cmd $argv[2..]
     if set -q __fish_data_dir[1]
-        $cmd $__fish_data_dir/$path
+        if not string match -rq -- ^/ $path
+            echo >&2 "error: $(status function) requires absolute paths on non-embedded builds"
+            return 1
+        end
+        $cmd $path
     else
         status get-file $path | $cmd
     end