This enables doing late modifications like updating the copyright notice or fixing line endings.

Also silence the checkout operations a bit more when VOID=subprocess2.VOID.

Add corresponding unit tests.

Review URL: http://codereview.chromium.org/6891003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@82587 0039d316-1c4b-4281-b951-d872f2087c98