parents: change parents command to use filectx

Changes the parents command to use filectx to look up the change node
instead of doing it manually.  This allows extensions to modify the
file-to-commit relationship behind the filectx api.

mercurial/commands.py

@@ -4356,8 +4356,10 b' def parents(ui, repo, file_=None, **opts'
                 pass
         if not filenodes:
             raise util.Abort(_("'%s' not found in manifest!") % file_)
-        fl = repo.file(file_)
-        p = [repo.lookup(fl.linkrev(fl.rev(fn))) for fn in filenodes]
+        p = []
+        for fn in filenodes:
+            fctx = repo.filectx(file_, fileid=fn)
+            p.append(fctx.node())
     else:
         p = [cp.node() for cp in ctx.parents()]