fix(refs): address literal branch rename review feedback

Review item #2274 (comment)
reports that "the returned head points to `refs/heads/topic`, which may be
missing or identify a different branch" after renaming to the short branch
name `refs/heads/topic`. A repository with an existing `topic` at commit A
and a source branch at commit B reproduces the incorrect returned ref on
both backends.

Compute the destination by prepending `refs/heads/` to the literal name
passed to `git branch`. Validate that same destination and retain it only
after Git successfully renames the branch. Git 2.54.0's
`builtin/branch.c::copy_or_rename_branch()` uses `refs.c::check_branch_ref()`,
which always prepends that namespace. The general `to_full_path()` helper
continues to normalize already-qualified references for its other callers.

The SHA-1/SHA-256 regressions fail before this fix and pass afterwards.
They check the returned path, name and commit, current `HEAD`, and that the
original `topic` still points to commit A. All 50 reference tests and 24
subtests pass on each backend, including ordinary and forced renames.
Ruff, ShellCheck, mypy and basedpyright pass.

Author: codex

git/refs/head.py

@@ -252,10 +252,11 @@ def rename(self, new_path: PathLike, force: bool = False) -> "Head":
             flag = "-M"
 
         new_name = Git._check_operand(new_path, "branch")
+        new_full_path = "%s/%s" % (self._common_path_default, new_name)
         self._get_validated_ref_path(self.repo, self.path)
-        self._get_validated_ref_path(self.repo, self.to_full_path(new_name))
+        self._get_validated_ref_path(self.repo, new_full_path)
         self.repo.git._call_process_safe("branch", flag, "--", self.name, new_name)
-        self.path = str(self.to_full_path(new_name))
+        self.path = new_full_path
         return self
 
     def checkout(

test/test_refs.py

@@ -92,6 +92,23 @@ def test_tag_without_tagger_initializes_all_metadata(object_format, message):
             assert tag.message == message
 
 
+@pytest.mark.parametrize("object_format", ["sha1", "sha256"])
+def test_head_rename_retains_literal_prefixed_branch_name(object_format):
+    with fixture_repo("refs.sh", "rename-prefixed", object_format, write=True) as repo:
+        original_topic = repo.heads.topic.commit
+        source = repo.head.reference
+        source_commit = source.commit
+        assert source_commit != original_topic
+        renamed = source.rename("refs/heads/topic")
+        assert renamed is source
+        assert renamed.path == "refs/heads/refs/heads/topic"
+        assert renamed.name == "refs/heads/topic"
+        assert renamed.commit == source_commit
+        assert repo.head.reference == renamed
+        assert repo.head.commit == source_commit
+        assert repo.heads.topic.commit == original_topic
+
+
 class TestRefs(TestBase):
     def test_from_path(self):
         # Should be able to create any reference directly.

tests/fixtures/refs.sh

@@ -30,6 +30,11 @@ case "$1" in
             fi
         } | git hash-object -w -t tag --literally --stdin >../tag-id
         ;;
+    rename-prefixed)
+        git branch topic
+        printf 'second\n' >> file.txt
+        git commit -am second
+        ;;
     outside)
         git rev-parse HEAD > ../outside-ref
         ;;