Crash when running git-bloom-generate from not-a-branch

Here is the trace:

```
Traceback (most recent call last):
  File "/usr/local/bin/git-bloom-generate", line 9, in <module>
    load_entry_point('bloom==0.2.16', 'console_scripts', 'git-bloom-generate')()
  File "/wg/stor5/pmathieu/devel/bloom/bloom/commands/git/generate.py", line 238, in main
    git_clone.commit()
  File "/wg/stor5/pmathieu/devel/bloom/bloom/git.py", line 94, in commit
    with inbranch(get_commit_hash(get_current_branch())):
  File "/wg/stor5/pmathieu/devel/bloom/bloom/git.py", line 340, in get_commit_hash
    cmd = 'git show-branch --sha1-name ' + reference
TypeError: cannot concatenate 'str' and 'NoneType' objects
```