revlog: filter out "invalid" delta base candidates

There are bases that we know won't produce a good delta chain. For example, if
the base is already bigger than twice the size of the text we store, we know
the resulting delta chain will never be valid.

We might make the check a bit more powerful and generic in the future, but
this looks a good start.

In particular, empty file (size 0) will never find a good base, so we should
stop spending time trying to find one.

mercurial/revlog.py

@@ -750,7 +750,11 b' class _deltacomputer(object):'
         deltaparent = self.revlog.deltaparent
 
         deltainfo = None
+        deltas_limit = revinfo.textlen * LIMIT_DELTA2TEXT
         for candidaterevs in self._getcandidaterevs(p1, p2, cachedelta):
+            # filter out delta base that will never produce good delta
+            candidaterevs = [r for r in candidaterevs
+                             if self.revlog.length(r) <= deltas_limit]
             nominateddeltas = []
             for candidaterev in candidaterevs:
                 # skip over empty delta (no need to include them in a chain)