Try switching to the branch of the same name, instead of detached head

Summary:
Landing from a branch that directly tracks origin/master places one in
a detached HEAD state. Instead, examine if there is a local branch of
the name that we landed onto, that also tracks the upstream; if so,
switch to that.

Test Plan:
Made a branch via git checkout -b testing origin/master
and tried to arc land

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T9723

Differential Revision: https://secure.phabricator.com/D14420