When applying a wildcard, it's important to keep track of the files that have
been visited, to avoid symlink loops. Previously fish used a FileId for the
purpose. However FileId also includes richer information like modification time;
thus if a file is modified during wildcard expansion then fish may believe that
the file is different and visit it twice.
The richer information like modification time is important for atomic file
writes but should be ignored for wildcard expansion; just use the (dev, inode)
pair instead.
This also somewhat reduces our reliance on struct stat, but we still need it for
fstatat which Rust does not expose.
