This provides env variable GCLIENT_KILL_GIT_FETCH_AFTER
that kills git fetch if it produces no output for that
many seconds.

Note that this is not final patch, but an experiment.
See http://crbug.com/635641#c24 for the deployment plan.

BUG=635641
R=hinoka@chromium.org

Review-Url: https://codereview.chromium.org/2241843002