experiments: fix checkpoints sorting issues in dvc exp show (#4740)

* experiments: order experiment (branches) by timestamp

* checkpoints: sort checkpoint branches by tip commit values

* remove sort logic from dvc.scm

dvc/command/experiments.py

@@ -155,7 +155,11 @@ def _sort_exp(experiments, sort_by, typ, reverse):
         ret = OrderedDict()
 
     def _sort(item):
-        _, exp = item
+        rev, exp = item
+        tip = exp.get("checkpoint_tip")
+        if tip and tip != rev:
+            # Sort checkpoint experiments by tip commit
+            return _sort((tip, experiments[tip]))
         for fname, item in exp.get(typ, {}).items():
             if isinstance(item, dict):
                 item = flatten(item)

dvc/repo/experiments/show.py

@@ -110,7 +110,12 @@ def show(
         )
 
     # collect reproduced experiments
-    for exp_branch in repo.experiments.scm.list_branches():
+    for head in sorted(
+        repo.experiments.scm.repo.heads,
+        key=lambda h: h.commit.committed_date,
+        reverse=True,
+    ):
+        exp_branch = head.name
         m = repo.experiments.BRANCH_RE.match(exp_branch)
         if m:
             rev = repo.scm.resolve_rev(m.group("baseline_rev"))