exp diff: Remove --old option.

After #6814 the header of `diff_table` is `a_rev/`b_rev` instead of `Old`/`New` so the name of the option was counterintuitive.

In addition, make `a_rev` show by default, for consistency with output format of other `diff_table` uses (`dvc metrics diff` / `dvc params diff`).

dvc/command/experiments.py

@@ -587,7 +587,6 @@ class CmdExperimentsDiff(CmdBase):
                     title=title,
                     markdown=self.args.markdown,
                     no_path=self.args.no_path,
-                    old=self.args.old,
                     on_empty_diff="diff not supported",
                     precision=precision if key == "metrics" else None,
                     a_rev=self.args.a_rev,
@@ -1114,12 +1113,6 @@ def add_parser(subparsers, parent_parser):
         dest="markdown",
         help="Show tabulated output in the Markdown format (GFM).",
     )
-    experiments_diff_parser.add_argument(
-        "--old",
-        action="store_true",
-        default=False,
-        help="Show old ('a_rev') metric/param value.",
-    )
     experiments_diff_parser.add_argument(
         "--no-path",
         action="store_true",

tests/unit/command/test_experiments.py

@@ -46,7 +46,6 @@ def test_experiments_diff(dvc, scm, mocker):
             "--param-deps",
             "--json",
             "--md",
-            "--old",
             "--precision",
             "10",
         ]
@@ -63,6 +62,29 @@ def test_experiments_diff(dvc, scm, mocker):
     )
 
 
+def test_experiments_diff_revs(mocker, capsys):
+    mocker.patch(
+        "dvc.repo.experiments.diff.diff",
+        return_value={
+            "params": {
+                "params.yaml": {"foo": {"diff": 1, "old": 1, "new": 2}}
+            },
+            "metrics": {
+                "metrics.yaml": {"foo": {"diff": 1, "old": 1, "new": 2}}
+            },
+        },
+    )
+
+    cli_args = parse_args(["exp", "diff", "exp_a", "exp_b"])
+    cmd = cli_args.func(cli_args)
+
+    capsys.readouterr()
+    assert cmd.run() == 0
+    cap = capsys.readouterr()
+    assert "exp_a" in cap.out
+    assert "exp_b" in cap.out
+
+
 def test_experiments_show(dvc, scm, mocker):
     cli_args = parse_args(
         [