After pushing at the end of "arc land", cascade the origin through all local tracking branches

Summary:
Fixes T9661. Users can construct arbitrarily long chains from the remote, like:

(remote) origin/master -> (local) cascade-a -> (local) cascade-b -> (local) cascade-c -> (local) cascade-d

When a user lands "cascade-d" onto "origin/master", we should pull A, B and C if they aren't ahead of the remote.

If a user lands "cascade-d" onto itself, we should pull A, B, and C if they aren't ahead of the remote, then reset D to the remote.

We also find this chain if the last component of it is connected by the local branch having the same name as the remote branch (typical for "master") instead of an actual connection through tracking brnaches.

Test Plan: See comment below.

Reviewers: chad

Reviewed By: chad

Subscribers: edibiase

Maniphest Tasks: T9661

Differential Revision: https://secure.phabricator.com/D14361