Support null base revisions in vcs.get_changed_files

src/taskgraph/util/vcs.py

@@ -505,6 +505,11 @@ def get_changed_files(self, diff_filter=None, mode=None, rev=None, base=None):
                 cmd.append("--cached")
             elif mode == "all":
                 cmd.append("HEAD")
+        elif base == self.NULL_REVISION:
+            # When base is NULL_REVISION (e.g new branches on Github), diff
+            # from the empty tree to show all files present at rev. Use Git's
+            # well-known empty tree hash.
+            cmd = ["diff", "4b825dc642cb6eb9a060e54bf8d69288fbee4904", rev]
         elif self.is_shallow:
             # In shallow clones, `git log` won't have the history necessary to
             # determine the files changed. Using `git diff` finds the

test/test_util_vcs.py

@@ -594,3 +594,57 @@ def test_get_changed_files_shallow_clone(git_repo, tmp_path, default_git_branch)
 
     modified = shallow_repo.get_changed_files("M", "all", feature_commit, main_commit)
     assert "file_to_modify.txt" in modified
+
+
+def test_get_changed_files_with_null_base_revision(repo):
+    second_file = os.path.join(repo.path, "second_file")
+    with open(second_file, "w") as f:
+        f.write("second file content")
+
+    repo.run("add", second_file)
+    repo.run("commit", "-m", "Add second file")
+
+    head_rev = repo.head_rev
+
+    changed_files = repo.get_changed_files(
+        "AMD", "all", rev=head_rev, base=Repository.NULL_REVISION
+    )
+
+    assert isinstance(changed_files, list)
+    # When base is NULL_REVISION, we should get all files from the beginning
+    # of history up to head_rev. This includes both the initial "first_file"
+    # and the newly added "second_file".
+    assert "first_file" in changed_files
+    assert "second_file" in changed_files
+
+
+def test_get_changed_files_with_null_base_revision_shallow_clone(
+    git_repo, tmp_path, default_git_branch
+):
+    tmp_repo = Path(git_repo)
+
+    (tmp_repo / "file1.txt").write_text("content 1")
+    (tmp_repo / "file2.txt").write_text("content 2")
+    subprocess.check_call(["git", "add", "."], cwd=tmp_repo)
+    subprocess.check_call(["git", "commit", "-m", "Add files"], cwd=tmp_repo)
+
+    commit_hash = subprocess.check_output(
+        ["git", "rev-parse", "HEAD"], cwd=tmp_repo, text=True
+    ).strip()
+
+    shallow_path = tmp_path / "shallow_null_test"
+    subprocess.check_call(
+        ["git", "clone", "--depth=1", f"file://{tmp_repo}", str(shallow_path)],
+        cwd=tmp_path,
+    )
+
+    shallow_repo = get_repository(str(shallow_path))
+    assert shallow_repo.is_shallow
+
+    changed_files = shallow_repo.get_changed_files(
+        "AMD", "all", rev=commit_hash, base=Repository.NULL_REVISION
+    )
+
+    assert "first_file" in changed_files
+    assert "file1.txt" in changed_files
+    assert "file2.txt" in changed_files