When cloning with `RepoBuilder`, specifying branch makes repository bare

I found a strange behavior in 0.13, not in the latest version 0.14. Since I couldn't find the CHANGELOG, I was not sure that the following bug was fixed intentionally, so I'm submitting an issue.

Here is a code that clones a remote repository on https using RepoBuilder.

use std::path::PathBuf;

use git2::{build::RepoBuilder};

fn main() {
    let mut builder = RepoBuilder::new();
    builder.branch("my-branch");   // <- this line
    let res = builder.clone("https://github.com/username/repo-name.git", &PathBuf::from("/tmp/x"));
    println!("{:?}", res.is_ok());
}

Without specifying builder (without "this line"), the repository is cloned normally (/tmp/x/ contains .git and the contents of the main branch). However, if the branch is specified (with "this line"), the behavior is as if git clone --bare was run: /tmp/x/ contains:

FETCH_HEAD   HEAD         config       description  hooks/       info/        objects/     refs/

Why specifying the branch makes repository bare? Is it a bug?

It seems I have libgit2 1.4.2 installed on the system. This behavior was confirmed only in 0.13.25; for 0.14.2, it didn't happen.

Start by reproducing the provided RepoBuilder::branch and clone example with git2-rs 0.13.25, then compare its behavior with 0.14.2 and the installed libgit2 1.4.2. Inspect the RepoBuilder branch and clone entry points; done means determining whether the bare repository result is an unintended version-specific bug or an intentional change.