sparse-revlog: fast-path before computing payload size

In this fast-path case, we do not need to compute the full delta chain
payload. Since it comes with a significant cost, we avoid doing so if
possible.

mercurial/revlogutils/deltas.py

@@ -257,13 +257,13 b' def _slicechunktodensity(revlog, revs, t'
         return
 
     deltachainspan = segmentspan(revlog, revs)
-    chainpayload = sum(length(r) for r in revs)
 
     if deltachainspan < mingapsize:
         yield revs
         return
 
     readdata = deltachainspan
+    chainpayload = sum(length(r) for r in revs)
 
     if deltachainspan:
         density = chainpayload / float(deltachainspan)