experiments: support exp run --checkpoint for baseline commits (#4701)

* handle untracked files when `git reset --hard` in experiments workspace

* error on checkpoint branch already exists

* terminate stage process on checkpoint errors

* update tests

* cleanup error message

tests/func/test_repro.py

@@ -1302,4 +1302,4 @@ def test_repro_when_cmd_changes(tmp_dir, dvc, run_copy, mocker):
         stage.addressing: ["changed checksum"]
     }
     assert dvc.reproduce(stage.addressing)[0] == stage
-    m.assert_called_once_with(stage, checkpoint=False)
+    m.assert_called_once_with(stage, checkpoint_func=None)

tests/func/test_repro_multistage.py

@@ -286,7 +286,7 @@ def test_repro_when_cmd_changes(tmp_dir, dvc, run_copy, mocker):
 
     assert dvc.status([target]) == {target: ["changed command"]}
     assert dvc.reproduce(target)[0] == stage
-    m.assert_called_once_with(stage, checkpoint=False)
+    m.assert_called_once_with(stage, checkpoint_func=None)
 
 
 def test_repro_when_new_deps_is_added_in_dvcfile(tmp_dir, dvc, run_copy):

tests/func/test_stage.py

@@ -288,4 +288,4 @@ def test_stage_run_checkpoint(tmp_dir, dvc, mocker, checkpoint):
     else:
         callback = None
     run_stage(stage, checkpoint_func=callback)
-    mock_cmd_run.assert_called_with(stage, checkpoint=checkpoint)
+    mock_cmd_run.assert_called_with(stage, checkpoint_func=callback)