fix(digest): rebaser must fetch feature branches with a full refspec

.github/scripts/fork_sync_digest.py

@@ -327,7 +327,10 @@ def git(*args, **kw):
             ["git", *args], cwd=repo_dir, capture_output=True, **kw
         )
 
-    fetched = git("fetch", "origin", branch)
+    # Explicit refspec: the clone is --single-branch, so a bare
+    # `fetch origin <branch>` only updates FETCH_HEAD, not
+    # refs/remotes/origin/<branch> — leaving the checkout below with no ref.
+    fetched = git("fetch", "origin", f"refs/heads/{branch}:refs/remotes/origin/{branch}")
     if fetched.returncode != 0:
         return row(500, "error", None, f"fetch failed: {fetched.stderr.decode()[:200]}")
 
@@ -336,7 +339,8 @@ def git(*args, **kw):
     if behind == 0:
         return row(200, "already-current", 0, "up to date with base")
 
-    git("checkout", "--detach", f"origin/{branch}", check=True)
+    # -f: discard the prior iteration's rebased worktree state cleanly.
+    git("checkout", "--detach", "-f", f"origin/{branch}", check=True)
     git("clean", "-fdq")
     mergebase = git("merge-base", base_ref, f"origin/{branch}").stdout.decode().strip()
     # Content-preserving rebase: hooks off (nothing new to lint — the branch

.github/scripts/tests/test_rebase_feature_branches.py

@@ -51,3 +51,35 @@ def boom(*a, **k):  # noqa: ANN002, ANN003
 
     monkeypatch.setattr(fsd.subprocess, "run", boom)
     assert fsd.rebase_feature_branches("ledoent/ddmrp", ["19.0-mig-*"], "19.0") == []
+
+
+class _Fake:
+    def __init__(self, rc=0, out=b"", err=b""):
+        self.returncode, self.stdout, self.stderr = rc, out, err
+
+
+def test_rebase_one_fetches_with_full_refspec(monkeypatch):
+    # Regression: a --single-branch clone + bare `fetch origin <branch>`
+    # populates FETCH_HEAD but NOT origin/<branch>, so the checkout below
+    # explodes. The fetch must use refs/heads/<b>:refs/remotes/origin/<b>.
+    from pathlib import Path
+
+    calls = []
+
+    def fake_run(cmd, cwd=None, capture_output=False, env=None, check=False, **k):
+        calls.append(cmd)
+        args = [c for c in cmd if c != "git"]
+        if "rev-list" in args:
+            return _Fake(0, out=b"2\n")
+        if "merge-base" in args:
+            return _Fake(0, out=b"abc123\n")
+        return _Fake(0)
+
+    monkeypatch.setattr(fsd.subprocess, "run", fake_run)
+    res = fsd._rebase_one(Path("/tmp/x"), "ledoent/ddmrp", "19.0-mig-foo", "19.0", "origin/19.0")
+    assert res["rebase_status"] == "rebased"
+    fetch_cmds = [c for c in calls if "fetch" in c]
+    assert fetch_cmds and any(
+        "refs/heads/19.0-mig-foo:refs/remotes/origin/19.0-mig-foo" in c
+        for c in fetch_cmds
+    ), f"fetch must use a full refspec, got: {fetch_cmds}"