tac: handle no line separators in file

Change the behavior of `tac` when there are no line separators in the
input. Previously, it was appending an extra line separator; this commit
prevents that from happening. The input is now writted directly to
stdout.

tests/by-util/test_tac.rs

@@ -68,3 +68,8 @@ fn test_invalid_input() {
         .fails()
         .stderr_contains("dir: read error: Invalid argument");
 }
+
+#[test]
+fn test_no_line_separators() {
+    new_ucmd!().pipe_in("a").succeeds().stdout_is("a");
+}