dvc: handle multiline command as list

Implements an approach proposed by @skshetry.

Fixes iterative#4373

dvc/stage/run.py

@@ -47,7 +47,7 @@ def warn_if_fish(executable):
 
 def _enforce_cmd_list(cmd):
     assert cmd
-    return cmd if isinstance(cmd, list) else [cmd]
+    return cmd if isinstance(cmd, list) else cmd.splitlines()
 
 
 def prepare_kwargs(stage, checkpoint_func=None):

tests/func/test_repro_multistage.py

@@ -529,7 +529,14 @@ def test_repro_multiple_params(tmp_dir, dvc):
     assert dvc.reproduce(stage.addressing) == [stage]
 
 
-def test_repro_list_of_commands_in_order(tmp_dir, dvc):
+@pytest.mark.parametrize("multiline", [True, False])
+def test_repro_list_of_commands_in_order(tmp_dir, dvc, multiline):
+    cmd = ["echo foo>foo", "echo bar>bar"]
+    if multiline:
+        cmd = "\n".join(cmd)
+
+    dump_yaml("dvc.yaml", {"stages": {"multi": {"cmd": cmd}}})
+
     (tmp_dir / "dvc.yaml").write_text(
         dedent(
             """\
@@ -546,19 +553,16 @@ def test_repro_list_of_commands_in_order(tmp_dir, dvc):
     assert (tmp_dir / "bar").read_text() == "bar\n"
 
 
-def test_repro_list_of_commands_raise_and_stops_after_failure(tmp_dir, dvc):
-    (tmp_dir / "dvc.yaml").write_text(
-        dedent(
-            """\
-            stages:
-              multi:
-                cmd:
-                - echo foo>foo
-                - failed_command
-                - echo baz>bar
-        """
-        )
-    )
+@pytest.mark.parametrize("multiline", [True, False])
+def test_repro_list_of_commands_raise_and_stops_after_failure(
+    tmp_dir, dvc, multiline
+):
+    cmd = ["echo foo>foo", "failed_command", "echo baz>bar"]
+    if multiline:
+        cmd = "\n".join(cmd)
+
+    dump_yaml("dvc.yaml", {"stages": {"multi": {"cmd": cmd}}})
+
     with pytest.raises(ReproductionError):
         dvc.reproduce(targets=["multi"])
     assert (tmp_dir / "foo").read_text() == "foo\n"