M1 R4: tracking_branch via git config (branch.<n>.remote/.merge) (#6)

src/repo/tracking_branch.rs — `tracking_branch(path) -> Result<Option<String>>` resolves the current branch's configured upstream to a short "remote/branch" name (e.g. "origin/main") from `branch.<name>.remote` + `.merge` via gix `config_snapshot().string()`; `Ok(None)` when no upstream or detached. Registered + soft-fail PyO3 wrapper. Parity-tested vs `git rev-parse --abbrev-ref @{upstream}` (with a real bare remote) and the no-upstream case.

PROVENANCE: implementation + tests written by the local model qwen2.5-coder:32b, driven
headlessly through `newt worker` (newt-agent's ACP worker) by the pilot, which
applied the module registration + PyO3 wrapper wiring and any clippy fixups.


Model: qwen2.5-coder:32b
Piloted-by: newt-agent

Co-authored-by: Shawn Hartsock <hartsock@users.noreply.github.com>
Co-authored-by: qwen2.5-coder:32b <model@newt.local>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

src/python.rs

@@ -63,8 +63,9 @@ fn current_branch(path: String) -> PyResult<Option<String>> {
 }
 
 #[pyfunction]
-fn tracking_branch(_path: String) -> PyResult<Option<String>> {
-    todo!("repo::tracking_branch (configured upstream)")
+fn tracking_branch(path: String) -> PyResult<Option<String>> {
+    // soft-fail: any error -> None (API.md)
+    Ok(crate::repo::tracking_branch(std::path::Path::new(&path)).unwrap_or(None))
 }
 
 #[pyfunction]

src/repo/mod.rs

@@ -29,6 +29,9 @@ pub use head_sha::head_sha;
 mod current_branch;
 pub use current_branch::current_branch;
 
+mod tracking_branch;
+pub use tracking_branch::tracking_branch;
+
 /// Temp-dir git fixtures shared by the per-method parity tests.
 ///
 /// Fixtures are built with the real `git` CLI, so each parity test asserts

src/repo/tracking_branch.rs

@@ -0,0 +1,62 @@
+use crate::error::GitxtendError;
+use crate::repo::Result;
+use std::path::Path;
+
+/// The configured upstream of the current branch as a short "remote/branch"
+/// name (e.g. "origin/main"); `Ok(None)` if there is no upstream or HEAD is
+/// detached. Mirrors `git rev-parse --abbrev-ref @{upstream}`.
+pub fn tracking_branch(path: &Path) -> Result<Option<String>> {
+    let repo = gix::open(path).map_err(GitxtendError::from_err)?;
+    let head = repo.head().map_err(GitxtendError::from_err)?;
+    let branch = match head.referent_name() {
+        Some(n) => n.shorten().to_string(), // e.g. "main"
+        None => return Ok(None),            // detached
+    };
+    let cfg = repo.config_snapshot();
+    // branch.<name>.remote  -> e.g. "origin"   ;  branch.<name>.merge -> "refs/heads/main"
+    let remote_key = format!("branch.{branch}.remote");
+    let merge_key = format!("branch.{branch}.merge");
+    let remote = cfg.string(remote_key.as_str()).map(|v| v.to_string());
+    let merge = cfg.string(merge_key.as_str()).map(|v| v.to_string());
+    match (remote, merge) {
+        (Some(r), Some(m)) => {
+            let short = m.strip_prefix("refs/heads/").unwrap_or(&m);
+            Ok(Some(format!("{r}/{short}")))
+        }
+        _ => Ok(None),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::repo::fixtures::{git, repo};
+
+    #[test]
+    fn no_upstream() {
+        let td = repo();
+        assert_eq!(tracking_branch(td.path()).unwrap(), None);
+    }
+
+    #[test]
+    fn with_upstream() {
+        let td = repo();
+        let remote = tempfile::tempdir().unwrap();
+        git(remote.path(), &["init", "--bare", "-q", "-b", "main"]);
+        git(
+            td.path(),
+            &["remote", "add", "origin", &remote.path().to_string_lossy()],
+        );
+        git(td.path(), &["push", "-q", "-u", "origin", "main"]);
+
+        let expected = Some("origin/main".into());
+        assert_eq!(tracking_branch(td.path()).unwrap(), expected);
+        assert_eq!(
+            tracking_branch(td.path()).unwrap(),
+            Some(git(
+                td.path(),
+                &["rev-parse", "--abbrev-ref", "@{upstream}"]
+            ))
+        );
+    }
+}