fix(gix): omit dangling symbolic references from enumeration

`for-each-ref` omits symbolic references whose targets do not exist, while
GixPython's reference iterator yields them. This made `Repo.heads` include
branches that the CLI backend would omit.

Resolve symbolic targets while retaining their original names. A dangling
target uses the existing read-error fallback, allowing Git to handle its
filtering and diagnostics. Valid symbolic aliases still use native enumeration.

The regression failed for both SHA-1 and SHA-256 before the fix. All six
selected enumeration, query, and reference tests now pass, as do Ruff lint
and format checks.

Author: codex

git/_backend.py

@@ -196,7 +196,12 @@ def _for_each_ref(repo: Any, args: List[str], kwargs: Dict[str, Any]) -> bytes:
     if any(char in prefix for char in b"*?["):
         raise _Unsupported("reference glob patterns")
     with repo.references().all() as refs:
-        names = [ref.name() for ref in refs]
+        names = []
+        for ref in refs:
+            names.append(ref.name())
+            if ref.target().try_name() is not None:
+                # Git omits dangling symbolic refs; defer their diagnostics to it.
+                ref.follow_to_object()
     return b"".join(
         name + b"\n"
         for name in sorted(names)

test/test_gix_backend.py

@@ -157,6 +157,19 @@ def test_per_command_committer_identity_uses_cli(repo, fields, monkeypatch):
         assert cli.call_count == 1
 
 
+def test_reference_enumeration_preserves_aliases_and_skips_dangling_refs(repo):
+    repo.index.commit("initial", skip_hooks=True)
+    repo.git.symbolic_ref("refs/heads/alias", "refs/heads/main")
+    with patch.object(Git, "execute", side_effect=AssertionError("unexpected CLI call")):
+        assert repo.git._call_process_safe("for_each_ref", "--format=%(refname)", "--", "refs/heads") == (
+            "refs/heads/alias\nrefs/heads/main"
+        )
+    repo.git.symbolic_ref("refs/heads/dangling", "refs/heads/missing")
+    with patch.object(_backend, "gix", None):
+        expected = [ref.path for ref in repo.heads]
+    assert [ref.path for ref in repo.heads] == expected == ["refs/heads/alias", "refs/heads/main"]
+
+
 def test_a_native_write_failure_is_not_retried(repo, monkeypatch):
     attempts = []