snapshot: turn _refinedgroups into a coroutine

We are now almost ready to start adding refining logic.

mercurial/revlogutils/deltas.py

@@ -589,7 +589,7 b' def _candidategroups(revlog, textlen, p1'
     tested = set([nullrev])
     candidates = _refinedgroups(revlog, p1, p2, cachedelta)
     while True:
-        temptative = next(candidates)
+        temptative = candidates.send(good)
         if temptative is None:
             break
         group = []
@@ -618,8 +618,6 b' def _candidategroups(revlog, textlen, p1'
             #      impacting performances. Some bounding or slicing mecanism
             #      would help to reduce this impact.
             good = yield tuple(group)
-        if good is not None:
-            break
     yield None
 
 def _findsnapshots(revlog, cache, start_rev):