Fix a few bugs in the userbenchmark abtest script (#940)
Summary:
Quit the script when git checkout failed, and fixed git checkout error handling.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/940
Reviewed By: davidberard98
Differential Revision: D36905371
Pulled By: xuzhao9
fbshipit-source-id: 02c99506ed5e4869e5daf9a05e1af4ef3ebdbee1
