dagutil: use revlog.parentrevs() for resolving parent revisions

dagutil: use revlog.parentrevs() for resolving parent revisions

And remove parents() since it is no longer used.

revlog.parentrevs() is almost the same as parents(). The main
difference is that parentrevs() can return nullrev. dagop.headrevs()
already handles nullrev. We add an inline check for nullrev in the
other call site to account for the difference.

.. api:: parents() removed from dagutil classes

   Use parentrevs() on the storage object instead.

Differential Revision: https://phab.mercurial-scm.org/D4328

--- a/mercurial/dagutil.py	Fri Aug 17 19:45:13 2018 +0000
+++ b/mercurial/dagutil.py	Fri Aug 17 19:48:52 2018 +0000
@@ -20,21 +20,6 @@
     def __init__(self, revlog):
         self._revlog = revlog
 
-    def parents(self, ix):
-        rlog = self._revlog
-        idx = rlog.index
-        revdata = idx[ix]
-        prev = revdata[5]
-        if prev != nullrev:
-            prev2 = revdata[6]
-            if prev2 == nullrev:
-                return [prev]
-            return [prev, prev2]
-        prev2 = revdata[6]
-        if prev2 != nullrev:
-            return [prev2]
-        return []
-
     def linearize(self, ixs):
         '''linearize and topologically sort a list of revisions
 
@@ -45,7 +30,7 @@
         parent, then adding the rev itself to the output list.
         '''
         sorted = []
-        visit = list(dagop.headrevs(ixs, self.parents))
+        visit = list(dagop.headrevs(ixs, self._revlog.parentrevs))
         visit.sort(reverse=True)
         finished = set()
 
@@ -58,7 +43,7 @@
                     finished.add(cur)
             else:
                 visit.append(-cur - 1)
-                visit += [p for p in self.parents(cur)
-                          if p in ixs and p not in finished]
+                visit += [p for p in self._revlog.parentrevs(cur)
+                          if p != nullrev and p in ixs and p not in finished]
         assert len(sorted) == len(ixs)
         return sorted