Revert D33827835: Try setting pull_request checkout to head ref
Test Plan: revert-hammer
Differential Revision:
D33827835 (https://github.com/pytorch/pytorch/commit/e27271d05a39bb51e43eec558dadedef2ebcf88f)
Original commit changeset: 45c7829f2ed8
Original Phabricator Diff: D33827835 (https://github.com/pytorch/pytorch/commit/e27271d05a39bb51e43eec558dadedef2ebcf88f)
fbshipit-source-id: 6b813c12dac54b6e3eea61afbf7a2b72e9bb2c67
