localrepo: reinstate localrepo.parents with a deprecation warning

The function was dropped in 3fe8cb40c9c5. This API drop brokes three of my
extensions including some critical to my workflow like tortoisehg. Lets mark
this API for death and give people time to fix their code.

mercurial/localrepo.py

@@ -851,6 +851,12 b' class localrepository(object):'
             f = f[1:]
         return filelog.filelog(self.svfs, f)
 
+    def parents(self, changeid=None):
+        '''get list of changectxs for parents of changeid'''
+        msg = 'repo.parents() is deprecated, use repo[%r].parents()' % changeid
+        self.ui.deprecwarn(msg, '3.7')
+        return self[changeid].parents()
+
     def changectx(self, changeid):
         return self[changeid]