subsetmaker: stabilize the computation of `scratch` subset

`heads` is set, order of the element are not deterministic and we need to
stabilize that if we want to get reproducible results.

Differential Revision: https://phab.mercurial-scm.org/D12394

contrib/perf-utils/subsetmaker.py

@@ -102,7 +102,9 b' def scratch(repo, subset, x):'
     for x in range(n):
         if not heads:
             break
-        pick = rand.choice(list(heads))
+        pickable = list(heads)
+        pickable.sort()
+        pick = rand.choice(pickable)
         heads.remove(pick)
         assert pick not in selected
         selected.add(pick)