fsext: Fix metadata_get_time for change time (ctime) before 1970

Useful for archaeologists and time travellers, do not crash if
ctime is before 1970.

Creating a file with such a change time is a bit challenging
(touch can't change that), so we can't easily add end-to-end
tests.

Steps to create a filesystem with a file `x` with birth/change
time in 1960.
```
dd if=/dev/zero bs=100M count=0 seek=1 of=ext4
mkfs.ext4 ext4
sudo mount ext4 mnt
touch mnt/x
umount mnt
echo "set_inode_field x ctime 196001010101" | debugfs -w ext4
echo "set_inode_field x ctime_extra 1234" | debugfs -w ext4
echo "set_inode_field x crtime 196001010101" | debugfs -w ext4
echo "set_inode_field x crtime_extra 0x123400" | debugfs -w ext4
sudo mount ext4 mnt
```

$ cargo run -p uu_stat mnt/x
  File: mnt/x
  size: 0         	Blocks: 0          IO Block: 1024   regular empty file
Device: 700h/1792d	Inode: 13          Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2025-07-26 19:52:00.849821694 +0800
Modify: 2025-07-26 19:52:00.849821694 +0800
Change: 1960-01-02 09:01:00.000298240 +0800
 Birth: 1960-01-02 09:01:00.000298240 +0800

Author: drinkcat

src/uucore/src/lib/features/fsext.rs

@@ -142,8 +142,20 @@ impl From<&str> for MetadataTimeField {
 
 #[cfg(unix)]
 fn metadata_get_change_time(md: &Metadata) -> Option<SystemTime> {
-    // TODO: This is incorrect for negative timestamps.
-    Some(UNIX_EPOCH + Duration::new(md.ctime() as u64, md.ctime_nsec() as u32))
+    let mut st = UNIX_EPOCH;
+    let (secs, nsecs) = (md.ctime(), md.ctime_nsec());
+    if secs >= 0 {
+        st += Duration::from_secs(secs as u64);
+    } else {
+        st -= Duration::from_secs(-secs as u64);
+    }
+    if nsecs >= 0 {
+        st += Duration::from_nanos(nsecs as u64);
+    } else {
+        // Probably never the case, but cover just in case.
+        st -= Duration::from_nanos(-nsecs as u64);
+    }
+    Some(st)
 }
 
 #[cfg(not(unix))]