wait-for-git only gets killed after a polling period

[flosell]

To be killable, wait-for-git checks if it was killed before polling. This works ok if we poll often. However, since we now have web-hook support (#6), we don't need to poll that often anymore.

Result:

• wait-for-git checks are in waiting state long after they were killed (e.g. because an either decided to do so)
• Potentially strange history and warnings when killed by shutdown (Implement clean shutdown lambdacd#103)

Solution:
Make kill-check independent of polling