This way the function is much more similar to subprocess.call(). Further changes
will be done, to be able to convert all the function, to be able to parallelize
output without corrupting it.

Used pylint to verify call graph correctness, causing some other unrelated
changes.

TEST=unit tests
BUG=none

Review URL: http://codereview.chromium.org/3117039

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@57369 0039d316-1c4b-4281-b951-d872f2087c98