build fails when introspecting git repository without a configured remote

[chuckbutler]

If i init a new layer, and haven't pushed/added a remote, the build process will error when trying to sniff the repository information.

$ charm build .
build: Composing into /home/ubuntu
bzr: ERROR: Not a branch: "/home/ubuntu/layers/juju-interfaces/".
Traceback (most recent call last):
  File "/usr/bin/charm-build", line 9, in <module>
    load_entry_point('charm-tools==2.0.0', 'console_scripts', 'charm-build')()
  File "/usr/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 645, in main
    build()
  File "/usr/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 497, in __call__
    self.validate()
  File "/usr/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 453, in validate
    self._validate_charm_repo()
  File "/usr/lib/python2.7/dist-packages/charmtools/build/__init__.py", line 485, in _validate_charm_repo
    recommended_repo = repofinder.get_recommended_repo(self.charm)
  File "/usr/lib/python2.7/dist-packages/charmtools/repofinder.py", line 38, in get_recommended_repo
    output = subprocess.check_output(cmd.args)
  File "/usr/lib/python2.7/subprocess.py", line 566, in check_output
    process = Popen(stdout=PIPE, *popenargs, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory