title: `pip freeze` breaks with package installation
body: <p>I am installing a package using dependency_links.  It seems to install the package alright but breaks pip freeze functionality (which probably means there is a deeper install issue.)  I want to be able to download the package from a custom server without any setup required outside of running <code>setup.py</code>.</p>
  
  <p>This is my <code>setup.py</code> file:</p>
  
  <pre><code>from setuptools import setup
  
    setup(
        name='package'
        ,version='0.1.0'
        ,packages=['foo',
                   'bar'
                   ]
        ,long_description=''
        ,url='https://github.com/myrepo'
        ,install_requires=['numpy&gt;=1.9.2'
                           ,'some_package'
                           ]
        ,dependency_links=[
            "http://custom_server/packages/some_package-0.1.0.tar.gz"
        ]               
    )
  </code></pre>
  
  <p>Install seems to work ok, but if I try and run <code>pip_freeze</code> I get the error below.  </p>
  
  <pre><code>pip freeze
  
  Error [Errno 20] Not a directory: '/Users/abc/anaconda/lib/python2.7/site-packages/some_package.egg' while executing command git rev-parse
  Exception:
  Traceback (most recent call last):
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/basecommand.py", line 209, in main
      status = self.run(options, args)
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/commands/freeze.py", line 70, in run
      for line in freeze(**freeze_kwargs):
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/operations/freeze.py", line 49, in freeze
      dependency_links
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/__init__.py", line 235, in from_dist
      if dist_is_editable(dist) and vcs.get_backend_name(location):
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/vcs/__init__.py", line 75, in get_backend_name
      if vc_type.controls_location(location):
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/vcs/git.py", line 266, in controls_location
      on_returncode='ignore')
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/vcs/__init__.py", line 322, in run_command
      spinner)
    File "/Users/myname/anaconda/lib/python2.7/site-packages/pip/utils/__init__.py", line 677, in call_subprocess
      cwd=cwd, env=env)
    File "/Users/myname/anaconda/lib/python2.7/subprocess.py", line 710, in __init__
      errread, errwrite)
    File "/Users/myname/anaconda/lib/python2.7/subprocess.py", line 1335, in _execute_child
      raise child_exception
  OSError: [Errno 20] Not a directory: '/Users/myname/anaconda/lib/python2.7/site-packages/some_package.egg'
  </code></pre>