Move overcommit-hook into template directory

In preparation for adding support for specifying a git template
directory, move the `overcommit-hook` master hook into a `template-dir`
directory.

Change-Id: If1ff4223140f81af4b71a9b7ef9002a975a5c477
Reviewed-on: http://gerrit.causes.com/35600
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>