Summary:
Previously we backed up extinct node if bookmark pointed to them. But this not
is not present in the bundle because `findcommonoutgoing()` intentionally skips
extinct and secret commits. In this case we'll have infinitepush bookmark that
points to non-existent infinitepush commit. To fix it let's filter extinct heads
Test Plan: arc unit
Reviewers: #mercurial
Subscribers: mjpieters, #sourcecontrol
Differential Revision: https://phabricator.intern.facebook.com/D4803322
Tasks: 15389402
