Add --allow-missing-config option to install

When no '.pre-commit-config.yaml' file exists while `pre-commit` hooks
are enabled, `pre-commit` returns an error and the action is aborted.
This is a very common scenario when pre-commit is added later on a
project and the user wants to work on a previous branch where the
configuration file does not exist.

This commits allow the user to optionally install the `pre-commit` hooks
with an option to allow a missing configuration and trigger only the
legacy pre-commit hooks (if any) when it is missing.

tests/commands/install_uninstall_test.py

@@ -26,6 +26,7 @@ from pre_commit.util import mkdirp
 from pre_commit.util import resource_filename
 from testing.fixtures import git_dir
 from testing.fixtures import make_consuming_repo
+from testing.fixtures import remove_config_from_repo
 from testing.util import cmd_output_mocked_pre_commit_home
 from testing.util import xfailif_no_symlink
 
@@ -64,7 +65,8 @@ def test_install_pre_commit(tempdir_factory):
     expected_contents = io.open(pre_commit_script).read().format(
         sys_executable=sys.executable,
         hook_type='pre-commit',
-        pre_push=''
+        pre_push='',
+        skip_on_missing_conf='false'
     )
     assert pre_commit_contents == expected_contents
     assert os.access(runner.pre_commit_path, os.X_OK)
@@ -79,6 +81,7 @@ def test_install_pre_commit(tempdir_factory):
         sys_executable=sys.executable,
         hook_type='pre-push',
         pre_push=pre_push_template_contents,
+        skip_on_missing_conf='false'
     )
     assert pre_push_contents == expected_contents
 
@@ -552,3 +555,46 @@ def test_pre_push_integration_empty_push(tempdir_factory):
         retc, output = _get_push_output(tempdir_factory)
         assert output == 'Everything up-to-date\n'
         assert retc == 0
+
+
+def test_install_disallow_mising_config(tempdir_factory):
+    path = make_consuming_repo(tempdir_factory, 'script_hooks_repo')
+    with cwd(path):
+        runner = Runner(path, C.CONFIG_FILE)
+
+        remove_config_from_repo(path)
+        assert install(runner, overwrite=True, skip_on_missing_conf=False) == 0
+
+        ret, output = _get_commit_output(tempdir_factory)
+        assert ret == 1
+
+
+def test_install_allow_mising_config(tempdir_factory):
+    path = make_consuming_repo(tempdir_factory, 'script_hooks_repo')
+    with cwd(path):
+        runner = Runner(path, C.CONFIG_FILE)
+
+        remove_config_from_repo(path)
+        assert install(runner, overwrite=True, skip_on_missing_conf=True) == 0
+
+        ret, output = _get_commit_output(tempdir_factory)
+        assert ret == 0
+        assert '`.pre-commit-config.yaml` config file not found. '\
+               'Skipping `pre-commit`.' in output
+
+
+def test_install_temporarily_allow_mising_config(tempdir_factory):
+    path = make_consuming_repo(tempdir_factory, 'script_hooks_repo')
+    with cwd(path):
+        runner = Runner(path, C.CONFIG_FILE)
+
+        remove_config_from_repo(path)
+        assert install(runner, overwrite=True, skip_on_missing_conf=False) == 0
+
+        extra_env = {'PRE_COMMIT_ALLOW_NO_CONFIG': '1'}
+        env = os.environ.copy()
+        env.update(extra_env)
+        ret, output = _get_commit_output(tempdir_factory, env=env)
+        assert ret == 0
+        assert '`.pre-commit-config.yaml` config file not found. '\
+               'Skipping `pre-commit`.' in output