A rename needs only a writable folder, so a read-only file would be
replaced where writing it in place would fail. Before staging, the
existing file is opened for writing without truncating; when that fails
the save fails with a read-only error.
The target is found by reading links one hop at a time, resolving
relative links against the link's folder. A dangling link writes the
file it names and stays a link, and a loop of links is an error.