Fix bug with multiple input objects to an external command.

Previously, we would build a command that looked something like this:

  <ex_cmd> "$it" "&&" "<ex_cmd>" "$it"

So that the "&&" and "<ex_cmd>" would also be arguments to the command,
instead of a chained command. This commit builds up a command string
that can be passed to an external shell.

tests/commands_test.rs

@@ -164,6 +164,28 @@ fn save_figures_out_intelligently_where_to_write_out_with_metadata() {
     })
 }
 
+#[test]
+fn it_arg_works_with_many_inputs_to_external_command() {
+    Playground::setup("it_arg_works_with_many_inputs", |dirs, sandbox| {
+        sandbox.with_files(vec![
+            FileWithContent("file1", "text"),
+            FileWithContent("file2", " and more text"),
+        ]);
+
+        let (stdout, stderr) = nu_combined!(
+            cwd: dirs.test(), h::pipeline(
+            r#"
+                echo file1 file2
+                | split-row " "
+                | cat $it
+            "#
+        ));
+
+        assert_eq!("text and more text", stdout);
+        assert!(!stderr.contains("No such file or directory"));
+    })
+}
+
 #[test]
 fn save_can_write_out_csv() {
     Playground::setup("save_test_2", |dirs, _| {