Summary:
hg backups would fail if it encountered a bundle whose parent wasn't present in
the repo anymore. This handles that case and prints out a message.
Test Plan:
Tried it in a large repo with lots of backup bundles, verified the
message was printed and the backups process continued on to the next one.
Reviewers: davidsp, pyd, rmcelroy, mitrandir, ericsumner, sid0
Reviewed By: sid0
Differential Revision: https://phabricator.fb.com/D1809904
Signature: t1:1809904:1422478920:90cff2103c4417e21da37e230e6058348f1c218a