Function branch_at 

pub fn branch_at(root: &Path) -> Option<String>

The branch the checkout at root has checked out, read from its HEAD file instead of asked of a subprocess. None when HEAD is detached, when the file cannot be read, or when root is not a checkout at all — the same three silences git symbolic-ref -q --short HEAD answers with, at the cost of at most two small reads.

It exists because a git subprocess is not free: on a loaded machine one costs tens of milliseconds, which is more than the whole rest of a narrow server.status reading. The linked-work-tree case is why it is not a single read — a linked checkout’s .git is a file naming the per-work-tree git directory, and HEAD lives there.

use majordomus_cli::repository::branch_at;
use std::process::Command;

let dir = tempfile::tempdir().unwrap();
let root = dir.path().join("repo");
std::fs::create_dir_all(&root).unwrap();
let git = |args: &[&str]| assert!(
    Command::new("git").arg("-C").arg(&root).args(args).status().unwrap().success()
);
git(&["init", "-q", "-b", "trunk", "."]);
assert_eq!(branch_at(&root).as_deref(), Some("trunk"), "the same name git would print");
assert!(branch_at(dir.path()).is_none(), "a plain directory has no branch");