git-hook: don't run when not in dvc repo (#4091)

Related to #4073

tests/unit/command/test_git_hook.py

@@ -0,0 +1,21 @@
+import pytest
+
+from dvc.cli import parse_args
+from dvc.command.git_hook import CmdPostCheckout, CmdPreCommit, CmdPrePush
+
+
+@pytest.mark.parametrize(
+    "hook, cls",
+    [
+        ("pre-commit", CmdPreCommit),
+        ("post-checkout", CmdPostCheckout),
+        ("pre-push", CmdPrePush),
+    ],
+)
+def test_out_of_repo(tmp_dir, hook, cls, mocker):
+    cli_args = parse_args(["git-hook", hook])
+    cli_args.func == cls
+    cmd = cli_args.func(cli_args)
+    mock_main = mocker.patch("dvc.main.main")
+    assert cmd.run() == 0
+    assert not mock_main.called