Rename head to tracking ref (apache#44585)

Head is confusing cus in git speak it means "what you have checked out".  That's not what we mean here.  Here we're trying to describe, most commonly, the branch in the repo that the user wants this bundle to track or "follow".  Branch would be a good name, but technically it could also be a tag, or even a commit hash.

airflow/dag_processing/bundles/git.py

@@ -38,26 +38,26 @@ class GitDagBundle(BaseDagBundle):
     and then do a clone for each version from there.
 
     :param repo_url: URL of the git repository
-    :param head: Branch or tag for this DAG bundle
+    :param tracking_ref: Branch or tag for this DAG bundle
     :param subdir: Subdirectory within the repository where the DAGs are stored (Optional)
     """
 
     supports_versioning = True
 
-    def __init__(self, *, repo_url: str, head: str, subdir: str | None = None, **kwargs) -> None:
+    def __init__(self, *, repo_url: str, tracking_ref: str, subdir: str | None = None, **kwargs) -> None:
         super().__init__(**kwargs)
         self.repo_url = repo_url
-        self.head = head
+        self.tracking_ref = tracking_ref
         self.subdir = subdir
 
         self.bare_repo_path = self._dag_bundle_root_storage_path / "git" / self.name
         self.repo_path = (
-            self._dag_bundle_root_storage_path / "git" / (self.name + f"+{self.version or self.head}")
+            self._dag_bundle_root_storage_path / "git" / (self.name + f"+{self.version or self.tracking_ref}")
         )
         self._clone_bare_repo_if_required()
         self._ensure_version_in_bare_repo()
         self._clone_repo_if_required()
-        self.repo.git.checkout(self.head)
+        self.repo.git.checkout(self.tracking_ref)
 
         if self.version:
             if not self._has_version(self.repo, self.version):

tests/dag_processing/test_dag_bundles.py

@@ -90,12 +90,12 @@ def test_supports_versioning(self):
 
     def test_uses_dag_bundle_root_storage_path(self, git_repo):
         repo_path, repo = git_repo
-        bundle = GitDagBundle(name="test", repo_url=repo_path, head="master")
+        bundle = GitDagBundle(name="test", repo_url=repo_path, tracking_ref="master")
         assert str(bundle._dag_bundle_root_storage_path) in str(bundle.path)
 
     def test_get_current_version(self, git_repo):
         repo_path, repo = git_repo
-        bundle = GitDagBundle(name="test", repo_url=repo_path, head="master")
+        bundle = GitDagBundle(name="test", repo_url=repo_path, tracking_ref="master")
 
         assert bundle.get_current_version() == repo.head.commit.hexsha
 
@@ -110,7 +110,9 @@ def test_get_specific_version(self, git_repo):
         repo.index.add([file_path])
         repo.index.commit("Another commit")
 
-        bundle = GitDagBundle(name="test", version=starting_commit.hexsha, repo_url=repo_path, head="master")
+        bundle = GitDagBundle(
+            name="test", version=starting_commit.hexsha, repo_url=repo_path, tracking_ref="master"
+        )
 
         assert bundle.get_current_version() == starting_commit.hexsha
 
@@ -132,7 +134,7 @@ def test_get_tag_version(self, git_repo):
         repo.index.add([file_path])
         repo.index.commit("Another commit")
 
-        bundle = GitDagBundle(name="test", version="test", repo_url=repo_path, head="master")
+        bundle = GitDagBundle(name="test", version="test", repo_url=repo_path, tracking_ref="master")
 
         assert bundle.get_current_version() == starting_commit.hexsha
 
@@ -149,7 +151,7 @@ def test_get_latest(self, git_repo):
         repo.index.add([file_path])
         repo.index.commit("Another commit")
 
-        bundle = GitDagBundle(name="test", repo_url=repo_path, head="master")
+        bundle = GitDagBundle(name="test", repo_url=repo_path, tracking_ref="master")
 
         assert bundle.get_current_version() != starting_commit.hexsha
 
@@ -160,7 +162,7 @@ def test_refresh(self, git_repo):
         repo_path, repo = git_repo
         starting_commit = repo.head.commit
 
-        bundle = GitDagBundle(name="test", repo_url=repo_path, head="master")
+        bundle = GitDagBundle(name="test", repo_url=repo_path, tracking_ref="master")
 
         assert bundle.get_current_version() == starting_commit.hexsha
 
@@ -184,14 +186,14 @@ def test_head(self, git_repo):
         repo_path, repo = git_repo
 
         repo.create_head("test")
-        bundle = GitDagBundle(name="test", repo_url=repo_path, head="test")
+        bundle = GitDagBundle(name="test", repo_url=repo_path, tracking_ref="test")
         assert bundle.repo.head.ref.name == "test"
 
     def test_version_not_found(self, git_repo):
         repo_path, repo = git_repo
 
         with pytest.raises(AirflowException, match="Version not_found not found in the repository"):
-            GitDagBundle(name="test", version="not_found", repo_url=repo_path, head="master")
+            GitDagBundle(name="test", version="not_found", repo_url=repo_path, tracking_ref="master")
 
     def test_subdir(self, git_repo):
         repo_path, repo = git_repo
@@ -206,7 +208,7 @@ def test_subdir(self, git_repo):
         repo.index.add([file_path])
         repo.index.commit("Initial commit")
 
-        bundle = GitDagBundle(name="test", repo_url=repo_path, head="master", subdir=subdir)
+        bundle = GitDagBundle(name="test", repo_url=repo_path, tracking_ref="master", subdir=subdir)
 
         files_in_repo = {f.name for f in bundle.path.iterdir() if f.is_file()}
         assert str(bundle.path).endswith(subdir)