Change how we apply lazy to the git hook

tests/unit/test_git.py

@@ -0,0 +1,29 @@
+"""Tests around functionality in the git integration."""
+import mock
+import pytest
+
+from flake8.main import git
+
+
+@pytest.mark.parametrize('lazy', [True, False])
+def test_find_modified_files(lazy):
+    """Confirm our logic for listing modified files."""
+    if lazy:
+        # Here --cached is missing
+        call = [
+            'git', 'diff-index', '--name-only', '--diff-filter=ACMRTUXB',
+            'HEAD'
+        ]
+    else:
+        call = [
+            'git', 'diff-index', '--cached', '--name-only',
+            '--diff-filter=ACMRTUXB', 'HEAD'
+        ]
+    mocked_popen = mock.Mock()
+    mocked_popen.communicate.return_value = ('', '')
+
+    with mock.patch('flake8.main.git.piped_process') as piped_process:
+        piped_process.return_value = mocked_popen
+        git.find_modified_files(lazy)
+
+    piped_process.assert_called_once_with(call)