mv: manages the 'seen' file list before moving

Should help with tests/mv/childproof.sh

tests/by-util/test_mv.rs

@@ -1469,6 +1469,32 @@ fn test_mv_file_into_dir_where_both_are_files() {
         .stderr_contains("mv: failed to access 'b/': Not a directory");
 }
 
+#[test]
+fn test_mv_seen_file() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.mkdir("a");
+    at.mkdir("b");
+    at.mkdir("c");
+    at.write("a/f", "a");
+    at.write("b/f", "b");
+
+    ts.ucmd()
+        .arg("a/f")
+        .arg("b/f")
+        .arg("c")
+        .fails()
+        .stderr_contains("will not overwrite just-created 'c/f' with 'b/f'");
+
+    // a/f has been moved into c/f
+    assert!(at.plus("c").join("f").exists());
+    // b/f still exists
+    assert!(at.plus("b").join("f").exists());
+    // a/f no longer exists
+    assert!(!at.plus("a").join("f").exists());
+}
+
 #[test]
 fn test_mv_dir_into_file_where_both_are_files() {
     let scene = TestScenario::new(util_name!());