bundle: when forcing acceptance of incoming delta also accept snapshot

Snapshot where never considered reusable and the unbundling side always tried
to find a delta from them. In the `forced` mode this is counter-productive
because it will either connect two delta-tree that should not be connected or
it will spend potentially a lot of time because creating a full snapshot
anyway.

So in this mode, we accept the full snapshot as is.

This changeset is benchmarked with its children so please do not split them
apart when landing.

mercurial/revlogutils/deltas.py

@@ -684,6 +684,15 b' def _candidategroups('
         yield None
         return
 
+    if (
+        cachedelta is not None
+        and nullrev == cachedelta[0]
+        and cachedelta[2] == DELTA_BASE_REUSE_FORCE
+    ):
+        # instruction are to forcibly do a full snapshot
+        yield None
+        return
+
     deltalength = revlog.length
     deltaparent = revlog.deltaparent
     sparse = revlog._sparserevlog