exp save: Support --message.

Custom commit message to use when committing the experiment.

Part of #8870

tests/func/experiments/test_save.py

@@ -153,3 +153,10 @@ def test_untracked_dvclock_is_included_in_exp(tmp_dir, scm, dvc):
     exp = dvc.experiments.save()
     fs = scm.get_fs(exp)
     assert fs.exists("dvc.lock")
+
+
+def test_exp_save_custom_message(tmp_dir, dvc, scm):
+    setup_stage(tmp_dir, dvc, scm)
+
+    exp = dvc.experiments.save(message="custom commit message")
+    assert scm.gitpython.repo.commit(exp).message == "custom commit message"

tests/unit/command/test_experiments.py

@@ -628,7 +628,7 @@ def test_experiments_save(dvc, scm, mocker):
     assert cmd.run() == 0
 
     m.assert_called_once_with(
-        cmd.repo, name="exp-name", force=True, include_untracked=[]
+        cmd.repo, name="exp-name", force=True, include_untracked=[], message=None
     )