delta-find: use a single snapshot cache when applying a group to an object

This will avoid walking the revlog over and over again in some situations.

The difference is hard to show in our current benchmark suite, as the gain is
lower than their overall instability.

mercurial/revlogutils/deltas.py

@@ -664,6 +664,7 b' def _candidategroups('
     cachedelta,
     excluded_bases=None,
     target_rev=None,
+    snapshot_cache=None,
 ):
     """Provides group of revision to be tested as delta base
 
@@ -689,6 +690,7 b' def _candidategroups('
         p1,
         p2,
         cachedelta,
+        snapshot_cache=snapshot_cache,
     )
     while True:
         temptative = candidates.send(good)
@@ -799,7 +801,7 b' def _candidategroups('
     yield None
 
 
-def _refinedgroups(revlog, p1, p2, cachedelta):
+def _refinedgroups(revlog, p1, p2, cachedelta, snapshot_cache=None):
     good = None
     # First we try to reuse a the delta contained in the bundle.
     # (or from the source revlog)
@@ -819,8 +821,8 b' def _refinedgroups(revlog, p1, p2, cache'
                 debug_info['cached-delta.accepted'] += 1
             yield None
             return
-    # XXX cache me higher
-    snapshot_cache = SnapshotCache()
+    if snapshot_cache is None:
+        snapshot_cache = SnapshotCache()
     groups = _rawgroups(
         revlog,
         p1,
@@ -1053,6 +1055,7 b' class deltacomputer:'
         self._write_debug = write_debug
         self._debug_search = debug_search
         self._debug_info = debug_info
+        self._snapshot_cache = SnapshotCache()
 
     def buildtext(self, revinfo, fh):
         """Builds a fulltext version of a revision
@@ -1265,6 +1268,7 b' class deltacomputer:'
             cachedelta,
             excluded_bases,
             target_rev,
+            snapshot_cache=self._snapshot_cache,
         )
         candidaterevs = next(groups)
         while candidaterevs is not None: