Hashstack should handle updates to hashdist better

[aterrel]

Error when running update and _hashdist is out of date:

$ ./update -v
Need to update Hashdist...
M       hashdist/core/run_job.py
Switched to branch 'master'
remote: Counting objects: 959, done.
remote: Compressing objects: 100% (426/426), done.
remote: Total 895 (delta 600), reused 754 (delta 465)
Receiving objects: 100% (895/895), 266.12 KiB | 0 bytes/s, done.
Resolving deltas: 100% (600/600), completed with 46 local objects.
From git://github.com/hashdist/hashdist
 * branch            master     -> FETCH_HEAD
   944564a..2bc9ce4  master     -> origin/master
Updating 944564a..2bc9ce4
error: Your local changes to the following files would be overwritten by merge:
        hashdist/core/run_job.py
Please, commit your changes or stash them before you can merge.
Aborting
Traceback (most recent call last):
  File "./update", line 7, in <module>
    setup(root_dir)
  File "/lustre/home1/u/aterrel/opt/apps/hashstack/master/builder/bootstrap.py", line 44, in setup
    fetch_hashdist(hashdist_path, hashdist_commit)
  File "/lustre/home1/u/aterrel/opt/apps/hashstack/master/builder/bootstrap.py", line 19, in fetch_hashdist
    subprocess.check_call(['git', 'pull', 'origin', 'master'], cwd=hashdist_path)
  File "/usr/lib64/python2.6/subprocess.py", line 488, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['git', 'pull', 'origin', 'master']' returned non-zero exit status 1