isatty() wasn't defined on Windows, causing it to crash.

R=dpranke@chromium.org
BUG=
TEST=manually gclient sync on windows


Review URL: http://codereview.chromium.org/8371006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@106865 0039d316-1c4b-4281-b951-d872f2087c98