df: implement the File column

Implement the "File" column in the `df` output table. Before this
commit, a blank entry appeared in the "File" column for each
row. After this commit, a "-" entry appears when `df` is run with no
positional arguments and the filename appears when run with positional
arguments. For example:

    $ touch a b c && df --output=target,file a b c
    Mounted on File
    /          a
    /          b
    /          c

tests/by-util/test_df.rs

@@ -222,4 +222,45 @@ fn test_output_selects_columns() {
     );
 }
 
-// ToDO: more tests...
+// TODO Fix the spacing.
+#[test]
+fn test_output_file_all_filesystems() {
+    // When run with no positional arguments, `df` lets "-" represent
+    // the "File" entry for each row.
+    let output = new_ucmd!()
+        .arg("--output=file")
+        .succeeds()
+        .stdout_move_str();
+    let mut lines = output.lines();
+    assert_eq!(lines.next().unwrap(), "File            ");
+    for line in lines {
+        assert_eq!(line, "-               ");
+    }
+}
+
+// TODO Fix the spacing.
+#[test]
+fn test_output_file_specific_files() {
+    // Create three files.
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.touch("a");
+    at.touch("b");
+    at.touch("c");
+
+    // When run with positional arguments, the filesystems should
+    // appear in the "File" column.
+    let output = ucmd
+        .args(&["--output=file", "a", "b", "c"])
+        .succeeds()
+        .stdout_move_str();
+    let actual: Vec<&str> = output.lines().collect();
+    assert_eq!(
+        actual,
+        vec![
+            "File            ",
+            "a               ",
+            "b               ",
+            "c               "
+        ]
+    );
+}