WalkSorter: Don't topo sort

Topo sorting slurps all interesting commits when the walk starts, and
the nature of this operation means we aren't marking any commits as
uninteresting. This makes WalkSorter far too slow for large
repositories; the default sort will have to be good enough.

Change-Id: I7d5b7e47aae10963a9787defd0067b68d7147b85

gerrit-server/src/test/java/com/google/gerrit/server/change/WalkSorterTest.java

@@ -115,12 +115,16 @@ public class WalkSorterTest {
 
     List<ChangeData> changes = ImmutableList.of(cd1, cd2, cd3, cd4);
     WalkSorter sorter = new WalkSorter(repoManager);
-
-    assertSorted(sorter, changes, ImmutableList.of(
+    List<PatchSetData> expected = ImmutableList.of(
         patchSetData(cd4, c4),
         patchSetData(cd3, c3),
         patchSetData(cd2, c2),
-        patchSetData(cd1, c1)));
+        patchSetData(cd1, c1));
+
+    for (List<ChangeData> list : permutations(changes)) {
+      // Not inOrder(); default sort can't distinguish between these.
+      assertThat(sorter.sort(list)).containsExactlyElementsIn(expected);
+    }
   }
 
   @Test