sparse-revlog: put the native implementation of slicechunktodensity to use

When possible, the C implementation of delta chain slicing will be used.
providing a large boost in performance for this operation.

To take a practical example of restoring manifest revision '59547c40bc4c' for
a reference NetBeans repository (using sparse-revlog). The media time of the
step `slice-sparse-chain` of `perfrevlogrevision` improve from 0.660 ms to
0.098 ms;

The full series move delta chain slicing from 1.120 ms to 0.098 ms;

Implementing _slicechunktosize into C would yield further improvements.
However, the performance seems good enough for now.

mercurial/revlogutils/deltas.py

@@ -44,6 +44,7 b' class _testrevlog(object):'
         self._srdensitythreshold = density
         self._srmingapsize = mingap
         self._snapshot = set(snapshot)
+        self.index = None
 
     def start(self, rev):
         if rev == 0:
@@ -120,9 +121,12 b' def slicechunk(revlog, revs, targetsize='
         targetsize = max(targetsize, revlog._srmingapsize)
     # targetsize should not be specified when evaluating delta candidates:
     # * targetsize is used to ensure we stay within specification when reading,
-    for chunk in _slicechunktodensity(revlog, revs,
-                                      revlog._srdensitythreshold,
-                                      revlog._srmingapsize):
+    densityslicing = getattr(revlog.index, 'slicechunktodensity', None)
+    if densityslicing is None:
+        densityslicing = lambda x, y, z: _slicechunktodensity(revlog, x, y, z)
+    for chunk in densityslicing(revs,
+                                revlog._srdensitythreshold,
+                                revlog._srmingapsize):
         for subchunk in _slicechunktosize(revlog, chunk, targetsize):
             yield subchunk