revlog: choose best delta for parentdelta (issue2466)

When parentdelta is enabled, we choose the delta that has the minimum
distance to its base. Otherwise, base may be sufficiently far away to
require a full version, resulting in greatly reduced compression.

mercurial/revlog.py

@@ -1194,15 +1194,14 b' class revlog(object):'
         d = None
         p1r, p2r = self.rev(p1), self.rev(p2)
 
-        if self._parentdelta:
-            deltarev, deltanode = p1r, p1
-            flags = REVIDX_PARENTDELTA
-        else:
-            deltarev, deltanode = prev, self.node(prev)
-
         # should we try to build a delta?
-        if deltarev != nullrev:
-            d = builddelta(deltarev)
+        if prev != nullrev:
+            d = builddelta(prev)
+            if self._parentdelta and prev != p1r:
+                d2 = builddelta(p1r)
+                if d2 < d:
+                    d = d2
+                    flags = REVIDX_PARENTDELTA
             dist, l, data, base = d
 
         # full versions are inserted when the needed deltas