sort: insert line separators after non-empty files

If files don't end witht a line separator we have to insert one,
otherwise the last line will be combined with the first line of the next
file.

tests/by-util/test_sort.rs

@@ -792,3 +792,11 @@ fn test_nonexistent_file() {
 fn test_blanks() {
     test_helper("blanks", &["-b", "--ignore-blanks"]);
 }
+
+#[test]
+fn sort_multiple() {
+    new_ucmd!()
+        .args(&["no_trailing_newline1.txt", "no_trailing_newline2.txt"])
+        .succeeds()
+        .stdout_is("a\nb\nb\n");
+}

tests/fixtures/sort/no_trailing_newline1.txt

@@ -0,0 +1,2 @@
+a
+b

tests/fixtures/sort/no_trailing_newline2.txt

@@ -0,0 +1 @@
+b