snapshot: try to refine new snapshot base down the chain

There are cases where doing a diff against a snapshot's parent will be shorter
than against the snapshot itself. Reusing snapshot not directly related to the
revision we are trying to store increase this odd.

So once we found a possible candidate, we check the snapshots lower in the
chain.

This will involve extra processing, but this extra processing will only happen
when we are doing building a snapshot, a rare situation.

mercurial/revlogutils/deltas.py

@@ -647,6 +647,17 b' def _refinedgroups(revlog, p1, p2, cache'
         good = yield candidates
         if good is not None:
             break
+
+    # if we have a refinable value, try to refine it
+    if good is not None and good not in (p1, p2) and revlog.issnapshot(good):
+        # refine snapshot down
+        previous = None
+        while previous != good:
+            previous = good
+            base = revlog.deltaparent(good)
+            if base == nullrev:
+                break
+            good = yield (base,)
     # we have found nothing
     yield None