Allow target.os to be a multivalue config option, which will cause
submodules required by any of the listed OSes to be checked out. You can
add additional OSes to the list with "git config --add target.os".

BUG=

Review URL: https://chromiumcodereview.appspot.com/13831004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@193901 0039d316-1c4b-4281-b951-d872f2087c98