presubmit test(bootstrap.py) doesn't properly check out PRs

[jlewi]

Presubmit test submitted on #36 failed with the following error.

INFO:root:Image info:
{
  "image": "gcr.io/mlkube-testing/builder:v20171017-470b0d6"
}
INFO:root:repo https://github.com/jlewi/mlkube.io.git
INFO:root:Running: git clone https://github.com/jlewi/mlkube.io.git /go/src/github.com/jlewi/mlkube.io
Cloning into '/go/src/github.com/jlewi/mlkube.io'...
INFO:root:Running: git checkout 9a62ee96cabfeb0a10fba304a634771325501e82
fatal: reference is not a tree: 9a62ee96cabfeb0a10fba304a634771325501e82
Traceback (most recent call last):
  File "/workspace/bootstrap.py", line 110, in <module>
    main()
  File "/workspace/bootstrap.py", line 102, in main
    src_dir, sha = clone_repo()
  File "/workspace/bootstrap.py", line 78, in clone_repo
    run(["git", "checkout", sha], cwd=dest)
  File "/workspace/bootstrap.py", line 38, in run
    subprocess.check_call(command, cwd=cwd)
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['git', 'checkout', '9a62ee96cabfeb0a10fba304a634771325501e82']' returned non-zero exit status 128

Looks like the issue is that bootstrap.py is trying to checkout the code from github.com/jlewi/mlkube.io. But since the PR is from a different user the code is in a different repo.

So to fix it I need to figure out which environment variables to use. We can modify boostrap.py to log the environment variables.

[jlewi]

Fixed by #60