Repository::is_empty() gives wrong answer for non-master initial branch

Summary

After initializing Git repository using a different initial branch name than master, the function git2::Repository::is_empty() will return false, even though there are no commits in any branch.

Steps to reproduce

// src/main.rs

use std::path::Path;
use std::process::{Command, Stdio};

fn main() {
    check_emptiness("main");    // will print repo is not empty
    check_emptiness("master");  // will print repo is empty
}

fn check_emptiness(branch: &str) {
    let dir = Path::new("/tmp/git2_test_dir").join(branch);
    if dir.exists() {
        std::fs::remove_dir_all(&dir).unwrap();
    }
    std::fs::create_dir_all(&dir).unwrap();
    run_command(&["git", "init", "--initial-branch", branch], &dir);
    let repo = git2::Repository::open(dir).unwrap();

    println!("is repo empty for branch {}? {}", branch, repo.is_empty().unwrap());
    let mut revision_walk = repo.revwalk().unwrap();
    revision_walk.push_head().unwrap_err();
}

fn run_command(command_parts: &[&str], dir: &Path)
{
    let mut command = Command::new(command_parts[0]);
    command.args(&command_parts[1..]).current_dir(&dir).stdout(Stdio::null());
    let status = command.status().unwrap();
    assert!(status.success(), "Exit code was: {:?}", status.code());
}


// Cargo.toml

[package]
name = "git2-empty-repository-test"
version = "0.1.0"
edition = "2018"

[dependencies]
git2 = "0.13.17"

Configuration

OS: Manjaro 20.2.1
Git: 2.30.0
git2: 0.13.17
rustc: 1.49.0

Notes

If you try to initialize Git repository with just git init, it will print a following warning:

hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint: 
hint:   git config --global init.defaultBranch <name>
hint: 
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint: 
hint:   git branch -m <name>

Start at the Repository::is_empty() entry point and reproduce the issue with the Rust program shown, comparing repositories initialized with main and master. Check why an initially branchless repository is treated differently, then verify that is_empty() reports true for both cases and that the revwalk behavior remains covered.