git-bloom-patch export does not check if directory exists after branch checkout

Error message:

```
⚪ git bloom-patch export
sh: 0: getcwd() failed: No such file or directory
fatal: Unable to read current working directory: No such file or directory
Traceback (most recent call last):
  File "/wg/stor5/pmathieu/repos/bloom/bloom/commands/git/patch/patch_main.py", line 49, in main
    retcode = args.func(args) or 0
  File "/wg/stor5/pmathieu/repos/bloom/bloom/commands/git/patch/export_cmd.py", line 85, in main
    return export_patches()
  File "/wg/stor5/pmathieu/repos/bloom/bloom/logging.py", line 164, in decorated
    return f(*args, **kwds)
  File "/wg/stor5/pmathieu/repos/bloom/bloom/commands/git/patch/export_cmd.py", line 65, in export_patches
    checkout(current_branch, directory=directory)
  File "/wg/stor5/pmathieu/repos/bloom/bloom/git.py", line 275, in checkout
    if reference == get_current_branch(directory):
  File "/wg/stor5/pmathieu/repos/bloom/bloom/git.py", line 584, in get_current_branch
    output = check_output(cmd, shell=True, cwd=directory)
  File "/wg/stor5/pmathieu/repos/bloom/bloom/util.py", line 281, in check_output
    raise CalledProcessError(p.returncode, cmd)
CalledProcessError: Command 'git branch --no-color' returned non-zero exit status 128
```