fn pick_watch_mount<'a>(mounts: &[&'a Path], target: &Path) -> Option<&'a Path>Expand description
Pick the mount whose path is the longest prefix of target.
Pure, and separated from Disks so it is testable without a real
filesystem. Longest-prefix rather than first-match because several mounts
legitimately prefix one path (/ and /home both prefix
/home/yas/.agentmux) and the most specific one is the filesystem whose
free space actually constrains a write there.