Hang when installing from git repository

[The-Compiler]

When trying to recreate an environment with a git repository in its dependencies, at least on Windows (haven't tested on Linux), tox hangs.

Looking at the logs with --verbose, it's because pip is waiting for a confirmation to nuke the old repo:

Obtaining PyInstaller from git+https://github.com/xoviat/pyinstaller.git@qtweb#egg=PyInstaller (from -r C:\Users\florian\qutebrowser/misc/requirements/requirements-pyinstaller.txt (line 3))
  git clone in c:\users\florian\qutebrowser\.tox\pyinstaller\src\pyinstaller exists with URL https://github.com/The-Compiler/qutebrowser.git
  The plan is to install the git repository https://github.com/xoviat/pyinstaller.git
What to do?  (s)witch, (i)gnore, (w)ipe, (b)ackup 

Involved versions:

pip (9.0.1)
pluggy (0.4.0)
py (1.4.33)
setuptools (28.8.0)
tox (2.6.0)
virtualenv (15.1.0)

[obestwalter]

Looks like we need to define the exists action option to make this go away.

[The-Compiler]

Nice find! FWIW adding install_command = pip install --exists-action w {opts} {packages} to the [testenv] section works as a workaround.