revlog: improve delta generation heuristics for generaldelta

Without this change, pulls (and clones) into a generaldelta repository could
generate very inefficient revlogs, the size of which could be at least twice
the original size.

This was caused by the generated delta chains covering too large distances,
causing new chains to be built far too often. This change addresses the
problem by forcing a delta against second parent or against the previous
revision, when the first parent delta is in danger of creating a long chain.

mercurial/revlog.py

@@ -1033,7 +1033,12 b' class revlog(object):'
         # should we try to build a delta?
         if prev != nullrev:
             if self._generaldelta:
-                d = builddelta(p1r)
+                if p1r >= basecache[1]:
+                    d = builddelta(p1r)
+                elif p2r >= basecache[1]:
+                    d = builddelta(p2r)
+                else:
+                    d = builddelta(prev)
             else:
                 d = builddelta(prev)
             dist, l, data, base, chainbase = d