sparse-revlog: use `span` variable as intended

The variable was planned to be used in the while condition but was not used
yet.

mercurial/revlogutils/deltas.py

@@ -264,7 +264,7 b' def _slicechunktosize(revlog, revs, targ'
             break # protect against individual chunk larger than limit
         localenddata = revlog.end(revs[endrevidx - 1])
         span = localenddata - startdata
-        while (localenddata - startdata) > targetsize:
+        while span > targetsize:
             if endrevidx - startrevidx <= 1:
                 break # protect against individual chunk larger than limit
             endrevidx -= (endrevidx - startrevidx) // 2