snapshot: extract parent chain computation

The final step of this series is to include chain related to "prev" in the
search. Before adding that code we do some simple code movement to clarify the
next diff.

mercurial/revlogutils/deltas.py

@@ -711,8 +711,9 b' def _rawgroups(revlog, p1, p2, cachedelt'
         # search for snapshot in parents delta chain
         # map: snapshot-level: snapshot-rev
         parents_snaps = collections.defaultdict(set)
-        for p in parents:
-            for idx, s in enumerate(deltachain(p)):
+        candidate_chains = [deltachain(p) for p in parents]
+        for chain in candidate_chains:
+            for idx, s in enumerate(chain):
                 if not revlog.issnapshot(s):
                     break
                 parents_snaps[idx].add(s)