hashsum: when the filename contains some special chars, escape them

Should fix tests/cksum/md5sum-newline.pl

tests/by-util/test_hashsum.rs

@@ -371,3 +371,18 @@ fn test_tag() {
             "SHA256 (foobar) = 1f2ec52b774368781bed1d1fb140a92e0eb6348090619c9291f9a5a3c8e8d151\n",
         );
 }
+
+#[test]
+#[cfg(not(windows))]
+fn test_with_escape_filename() {
+    let scene = TestScenario::new(util_name!());
+
+    let at = &scene.fixtures;
+    let filename = "a\nb";
+    at.touch(filename);
+    let result = scene.ccmd("md5sum").arg("--text").arg(filename).succeeds();
+    let stdout = result.stdout_str();
+    println!("stdout {}", stdout);
+    assert!(stdout.starts_with('\\'));
+    assert!(stdout.trim().ends_with("a\\nb"));
+}