Failed import raises non-ImportError exception on MacOS

[terminalmage]

This is related to #657.

Apparently, MacOS adds a git executable that is just a stub which displays an error. This gets past the try/except I added in #658, and allows all of the GitPython components to be imported, but since the executable is not actually git, it results in an exception when refresh() attemepts to run a git version:

  File "/opt/salt/lib/python2.7/site-packages/salt/utils/gitfs.py", line 90, in <module>
    import git
  File "/opt/salt/lib/python2.7/site-packages/git/__init__.py", line 82, in <module>
    refresh()
  File "/opt/salt/lib/python2.7/site-packages/git/__init__.py", line 73, in refresh
    if not Git.refresh(path=path):
  File "/opt/salt/lib/python2.7/site-packages/git/cmd.py", line 230, in refresh
    cls().version()
  File "/opt/salt/lib/python2.7/site-packages/git/cmd.py", line 551, in <lambda>
    return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
  File "/opt/salt/lib/python2.7/site-packages/git/cmd.py", line 1010, in _call_process
    return self.execute(call, **exec_kwargs)
  File "/opt/salt/lib/python2.7/site-packages/git/cmd.py", line 821, in execute
    raise GitCommandError(command, status, stderr_value, stdout_value)
GitCommandError: Cmd('git') failed due to: exit code(1)
  cmdline: git version
  stderr: 'xcode-select: error: no developer tools were found at '/Applications/Xcode.app', and no install could be requested (perhaps no UI is present), please install manually from 'developer
.apple.com'.'

Fix incoming.

[terminalmage]

Fix in #763.