snapshot: also use None as a stop value for `_refinegroup`

This is yet another small step toward turning `_refinegroups` into a co-routine.

mercurial/revlogutils/deltas.py

@@ -587,7 +587,11 b' def _candidategroups(revlog, textlen, p1'
     deltas_limit = textlen * LIMIT_DELTA2TEXT
 
     tested = set([nullrev])
-    for temptative in _refinedgroups(revlog, p1, p2, cachedelta):
+    candidates = _refinedgroups(revlog, p1, p2, cachedelta)
+    while True:
+        temptative = next(candidates)
+        if temptative is None:
+            break
         group = []
         for rev in temptative:
             # skip over empty delta (no need to include them in a chain)
@@ -632,6 +636,8 b' def _refinedgroups(revlog, p1, p2, cache'
         good = yield candidates
         if good is not None:
             break
+    # we have found nothing
+    yield None
 
 def _rawgroups(revlog, p1, p2, cachedelta):
     """Provides group of revision to be tested as delta base