revlog: move the `deltachain` method on the inner object

This is a necessary step before being able to move more logic around restoring
a revision content there.

For now, we do a simple patch for the perf extension logic, when the
implementation of the inner object changes, we will likely need some evolution
of the API. However this is true of many things in the perf extension. So we
will see this later.

mercurial/revlog.py

@@ -460,6 +460,47 b' class _InnerRevlog:'
             return False
         return self.issnapshot(base)
 
+    def _deltachain(self, rev, stoprev=None):
+        """Obtain the delta chain for a revision.
+
+        ``stoprev`` specifies a revision to stop at. If not specified, we
+        stop at the base of the chain.
+
+        Returns a 2-tuple of (chain, stopped) where ``chain`` is a list of
+        revs in ascending order and ``stopped`` is a bool indicating whether
+        ``stoprev`` was hit.
+        """
+        generaldelta = self.delta_config.general_delta
+        # Try C implementation.
+        try:
+            return self.index.deltachain(rev, stoprev, generaldelta)
+        except AttributeError:
+            pass
+
+        chain = []
+
+        # Alias to prevent attribute lookup in tight loop.
+        index = self.index
+
+        iterrev = rev
+        e = index[iterrev]
+        while iterrev != e[3] and iterrev != stoprev:
+            chain.append(iterrev)
+            if generaldelta:
+                iterrev = e[3]
+            else:
+                iterrev -= 1
+            e = index[iterrev]
+
+        if iterrev == stoprev:
+            stopped = True
+        else:
+            chain.append(iterrev)
+            stopped = False
+
+        chain.reverse()
+        return chain, stopped
+
     @util.propertycache
     def _compressor(self):
         engine = util.compengines[self.feature_config.compression_engine]
@@ -1003,7 +1044,6 b' class revlog:'
 
         chunk_cache = self._loadindex()
         self._load_inner(chunk_cache)
-
         self._concurrencychecker = concurrencychecker
 
     @property
@@ -1823,45 +1863,7 b' class revlog:'
         return r
 
     def _deltachain(self, rev, stoprev=None):
-        """Obtain the delta chain for a revision.
-
-        ``stoprev`` specifies a revision to stop at. If not specified, we
-        stop at the base of the chain.
-
-        Returns a 2-tuple of (chain, stopped) where ``chain`` is a list of
-        revs in ascending order and ``stopped`` is a bool indicating whether
-        ``stoprev`` was hit.
-        """
-        generaldelta = self.delta_config.general_delta
-        # Try C implementation.
-        try:
-            return self.index.deltachain(rev, stoprev, generaldelta)
-        except AttributeError:
-            pass
-
-        chain = []
-
-        # Alias to prevent attribute lookup in tight loop.
-        index = self.index
-
-        iterrev = rev
-        e = index[iterrev]
-        while iterrev != e[3] and iterrev != stoprev:
-            chain.append(iterrev)
-            if generaldelta:
-                iterrev = e[3]
-            else:
-                iterrev -= 1
-            e = index[iterrev]
-
-        if iterrev == stoprev:
-            stopped = True
-        else:
-            chain.append(iterrev)
-            stopped = False
-
-        chain.reverse()
-        return chain, stopped
+        return self._inner._deltachain(rev, stoprev=stoprev)
 
     def ancestors(self, revs, stoprev=0, inclusive=False):
         """Generate the ancestors of 'revs' in reverse revision order.
@@ -2496,7 +2498,7 b' class revlog:'
         """number of snapshot in the chain before this one"""
         if not self.issnapshot(rev):
             raise error.ProgrammingError(b'revision %d not a snapshot')
-        return len(self._deltachain(rev)[0]) - 1
+        return len(self._inner._deltachain(rev)[0]) - 1
 
     def revdiff(self, rev1, rev2):
         """return or calculate a delta between two revisions
@@ -2594,7 +2596,7 b' class revlog:'
         if rev is None:
             rev = self.rev(node)
 
-        chain, stopped = self._deltachain(rev, stoprev=cachedrev)
+        chain, stopped = self._inner._deltachain(rev, stoprev=cachedrev)
         if stopped:
             basetext = self._revisioncache[2]