Resolves https://github.com/nushell/nushell/issues/1658 (#1660)

For example, when running the following:

    crates/nu-cli/src

nushell currently parses this as an external command. Before running the command, we check to see if
it's a directory. If it is, we "auto cd" into that directory, otherwise we go through normal
external processing.

If we put a trailing slash on it though, shells typically interpret that as "user is explicitly
referencing directory". So

    crates/nu-cli/src/

should not be interpreted as "run an external command". We intercept a trailing slash in the head
position of a command in a pipeline as such, and inject a `cd` internal command.

tests/shell/pipeline/commands/external.rs

@@ -25,7 +25,7 @@ fn automatically_change_directory() {
     use nu_test_support::playground::Playground;
 
     Playground::setup("cd_test_5_1", |dirs, sandbox| {
-        sandbox.within("autodir").mkdir("bar");
+        sandbox.mkdir("autodir");
 
         let actual = nu!(
             cwd: dirs.test(),
@@ -39,6 +39,25 @@ fn automatically_change_directory() {
     })
 }
 
+#[test]
+fn automatically_change_directory_with_trailing_slash_and_same_name_as_command() {
+    use nu_test_support::playground::Playground;
+
+    Playground::setup("cd_test_5_1", |dirs, sandbox| {
+        sandbox.mkdir("cd");
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            r#"
+                cd/
+                pwd | echo $it
+            "#
+        );
+
+        assert!(actual.ends_with("cd"));
+    })
+}
+
 mod it_evaluation {
     use super::nu;
     use nu_test_support::fs::Stub::{EmptyFile, FileWithContent, FileWithContentToBeTrimmed};