Error when call_subprocess fails with show_stdout=True

[srinchiera]

If call_subprocess in utils/__init__.py is called with show_stdout=True, and the subprocess fails, pip errors out with UnboundLocalError: local variable 'all_output' referenced before assignment.

To avoid this, it should not try to print all_output when called when show_stdout=True. In this case, the process's stdout will already be printed to the console.

Discovered using this command:

$ pip install git+ssh://git@github.com:uber/vertica-python.git@0.2.1
Collecting git+ssh://git@github.com:uber/vertica-python.git@0.2.1
  Cloning ssh://git@github.com:uber/vertica-python.git (to 0.2.1) to /tmp/pip-8k63_T-build
ssh: Could not resolve hostname github.com:uber: Name or service not known
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
git clone -q ssh://git@github.com:uber/vertica-python.git /tmp/pip-8k63_T-build
  Complete output from command git clone -q ssh://git@github.com:uber/vertica-python.git /tmp/pip-8k63_T-build:
Exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/pip/basecommand.py", line 209, in main
    status = self.run(options, args)
  File "/usr/lib/python2.7/site-packages/pip/commands/install.py", line 299, in run
    requirement_set.prepare_files(finder)
  File "/usr/lib/python2.7/site-packages/pip/req/req_set.py", line 359, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/usr/lib/python2.7/site-packages/pip/req/req_set.py", line 576, in _prepare_file
    session=self.session, hashes=hashes)
  File "/usr/lib/python2.7/site-packages/pip/download.py", line 793, in unpack_url
    unpack_vcs_link(link, location)
  File "/usr/lib/python2.7/site-packages/pip/download.py", line 474, in unpack_vcs_link
    vcs_backend.unpack(location)
  File "/usr/lib/python2.7/site-packages/pip/vcs/__init__.py", line 283, in unpack
    self.obtain(location)
  File "/usr/lib/python2.7/site-packages/pip/vcs/git.py", line 124, in obtain
    self.run_command(['clone', '-q', url, dest])
  File "/usr/lib/python2.7/site-packages/pip/vcs/__init__.py", line 322, in run_command
    spinner)
  File "/usr/lib/python2.7/site-packages/pip/utils/__init__.py", line 712, in call_subprocess
    ''.join(all_output) +
UnboundLocalError: local variable 'all_output' referenced before assignment