Don't follow symlinks in append_dir_all() if follow_symlinks(false)

Before (append_dir_all("file")):
  called `Result::unwrap()` on an `Err` value: Os { code: 20, kind: NotADirectory, message: "Not a directory" }
  called `Result::unwrap()` on an `Err` value: Os { code: 267, kind: NotADirectory, message: "The directory name is invalid." }
after (append_dir_all("file") or append_dir_all("symlink"):
  called `Result::unwrap()` on an `Err` value: Custom { kind: NotADirectory, error: "append_dir_all() argument not a directory" }

Repro with
  fn main() {
      let mut tar = tar::Builder::new(std::io::stdout().lock());
      tar.follow_symlinks(false);
      tar.append_dir_all("", std::env::args_os().nth(1).unwrap()) .unwrap();
      tar.finish().unwrap();
  }
before:
  $ ls -lnd src symlink Cargo.toml
  -rw-r--r-- 1 1000 100 118 10-08 18:31 Cargo.toml
  drwxr-xr-x 2 1000 100  60 10-08 17:57 src
  lrwxrwxrwx 1 1000 100   3 10-08 18:30 symlink -> src
  $ cargo run -- Cargo.toml | tar -tv
  called `Result::unwrap()` on an `Err` value: Os { code: 20, kind: NotADirectory, message: "Not a directory" }
  $ cargo run -- src | tar -tv
  -rw-r--r-- 1000/100        211 2025-10-08 18:29 main.rs
  $ cargo run -- symlink | tar -tv
  -rw-r--r-- 1000/100        211 2025-10-08 18:29 main.rs
after:
  $ cargo run -- Cargo.toml | tar -tv
  called `Result::unwrap()` on an `Err` value: Custom { kind: NotADirectory, error: "append_dir_all() argument not a directory" }
  $ cargo run -- src | tar -tv
  -rw-r--r-- 1000/100        211 2025-10-08 18:29 main.rs
  $ cargo run -- symlink | tar -tv
  called `Result::unwrap()` on an `Err` value: Custom { kind: NotADirectory, error: "append_dir_all() argument not a directory" }

This also achieves parity with tar(1)

Author: nabijaczleweli

src/builder.rs

@@ -870,7 +870,18 @@ fn append_dir_all(
     src_path: &Path,
     options: BuilderOptions,
 ) -> io::Result<()> {
-    let mut stack = vec![(src_path.to_path_buf(), true, false)];
+    let src_metadata = if options.follow {
+        fs::metadata(src_path)
+    } else {
+        fs::symlink_metadata(src_path)
+    }?;
+    if !src_metadata.is_dir() {
+        return Err(io::Error::new(
+            io::ErrorKind::NotADirectory,
+            "append_dir_all() argument not a directory",
+        ));
+    }
+    let mut stack = vec![(src_path.to_path_buf(), true, src_metadata.is_symlink())];
     while let Some((src, is_dir, is_symlink)) = stack.pop() {
         let dest = path.join(src.strip_prefix(src_path).unwrap());
         // In case of a symlink pointing to a directory, is_dir is false, but src.is_dir() will return true