revlog: early return in _slicechunk when density is already good

We don't need to do anything if we know we won't enter the second while loop.
Save the overhead of the first loop by returning earlier.

mercurial/revlog.py

@@ -234,6 +234,10 b' def _slicechunk(revlog, revs):'
     else:
         density = 1.0
 
+    if density >= revlog._srdensitythreshold:
+        yield revs
+        return
+
     # Store the gaps in a heap to have them sorted by decreasing size
     gapsheap = []
     heapq.heapify(gapsheap)