_autoenv_get_file_upwards: do not dereference symlinks (#73)
This prevents issues where autoenv scripts use $0. When a shell enters the directory holding the autoenv scripts, it works as expected with $0 being the path to the symlink. However, if the shell enters one of its child directories the path to the script is dereferenced, and $0 is instead the path to the symlink's target.
From `man 1 zshexpn`:
> A – Turn a file name into an absolute path as the 'a' modifier does, and then pass the result through the realpath(3) library function to resolve symbolic links.
Note: Symlinks are dereferenced elsewhere for authorization, so that behavior is unchanged.