Taken from GNU realpath, this one makes realpath not resolve symlinks.
It still makes paths absolute and handles duplicate and trailing
slashes.
(useful in fish_add_path)
I'm gonna add more tests to this and I don't want to touch the old stuff.
Notice that this needs to have the output of the complete_directories
test adjusted because this one now runs later.
That's something we should take into account in future.
