Allow running on detached head

hooks/utils.py

@@ -11,5 +11,7 @@ def get_untracked_files() -> List[str]:
 
 
 def get_current_branch() -> str:
-    output = subprocess.check_output(BRANCH_CMD)
-    return output.decode().rstrip()
+    result = subprocess.run(BRANCH_CMD, stdout=subprocess.PIPE)
+    if result.returncode == 0:
+        return result.stdout.decode().rstrip()
+    return None

tests/check_untracked_migrations_test.py

@@ -25,3 +25,13 @@ def test_running_on_correct_branch(temp_git_dir):
 def test_running_on_incorrect_branch(temp_git_dir):
     with temp_git_dir.as_cwd():
         assert main(["--branches", "branch_one", "branch_two"]) == 1
+
+
+def test_running_on_detached_head_no_filter(temp_git_detached_dir):
+    with temp_git_detached_dir.as_cwd():
+        assert main() == 0
+
+
+def test_running_on_detached_head_with_filter(temp_git_detached_dir):
+    with temp_git_detached_dir.as_cwd():
+        assert main(["--branches", "branch_one", "branch_two"]) == 1

tests/conftest.py

@@ -8,3 +8,14 @@ def temp_git_dir(tmpdir):
     git_dir = tmpdir.join("gits")
     subprocess.call(["git", "init", "--", str(git_dir)])
     yield git_dir
+
+
+@pytest.fixture
+def temp_git_detached_dir(tmpdir):
+    git_dir = tmpdir.join("gits")
+    subprocess.call(["git", "init", "--", str(git_dir)])
+    subprocess.call(
+        ["git", "commit", "--allow-empty", "-m", "initial commit"], cwd=git_dir
+    )
+    subprocess.call(["git", "checkout", "--detach"], cwd=git_dir)
+    yield git_dir