sparse-revlog: fast-path before computing payload size

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.

--- a/mercurial/revlogutils/deltas.py	Tue Oct 16 14:46:28 2018 +0200
+++ b/mercurial/revlogutils/deltas.py	Thu Nov 08 15:29:58 2018 +0100
@@ -257,13 +257,13 @@
         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)