addrevision: handle code path not producing delta

We would like to be able to exit the delta generation block without a valid
delta (for a more flexible control flow). So we make sure we do not expand the
"delta" content unless we actually have a delta.

We can do it one level lower because 'delta' is initialised at None anyway. Not
adding a level to the assignment prevent a line length issue.

mercurial/revlog.py

@@ -1459,6 +1459,7 b' class revlog(object):'
                         delta = builddelta(prev)
             else:
                 delta = builddelta(prev)
+        if delta is not None:
             dist, l, data, base, chainbase, chainlen, compresseddeltalen = delta
 
         if not self._isgooddelta(delta, textlen):