This patch modifies our PR merge script to reset back to a named branch when restoring the original checkout upon exit. When the committer is originally checked out to a detached head, then they will be restored back to that same ref (the same as today's behavior).

This is a slightly updated version of #7569, with an extra fix to handle the detached head corner-case.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #10709 from JoshRosen/SPARK-9383.