Exception on --download on git/hg URL without egg name

[vilcans]

This fails:

$ pip install --download=downloaded/ 'git+https://github.com/fhahn/django-permission-backend-nonrel'
Downloading/unpacking git+https://github.com/fhahn/django-permission-backend-nonrel
  Cloning https://github.com/fhahn/django-permission-backend-nonrel to /tmp/pip-QNLCYt-export
Unpacking objects: 100% (51/51), done.
Exception:
Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/pip-1.0.2-py2.6.egg/pip/basecommand.py", line 126, in main
    self.run(options, args)
  File "/usr/local/lib/python2.6/dist-packages/pip-1.0.2-py2.6.egg/pip/commands/install.py", line 235, in run
    requirement_set.successfully_downloaded])
TypeError: sequence item 0: expected string, NoneType found

..but if I supply an egg name, it seems to work:

$ pip install --download=downloaded/ 'git+https://github.com/fhahn/django-permission-backend-nonrel#egg=django-permission-nonrel'
Downloading/unpacking django-permission-nonrel from git+https://github.com/fhahn/django-permission-backend-nonrel
  Cloning https://github.com/fhahn/django-permission-backend-nonrel to /tmp/pip-8WzAcF-export
Unpacking objects: 100% (51/51), done.
Successfully downloaded django-permission-nonrel

...but there is no file downloaded into the downloaded directory.

[xavfernandez]

Duplicate of #429 and fixed in latest pip.