Add commit hook and make the verification customizable.

Add commit hook and make the verification customizable.

There are three hooks:

    - 'pre-commit' is given an opportunity to inspect what is
      being committed, before we invoke the EDITOR for the
      commit message;

    - 'commit-msg' is invoked on the commit log message after
      the user prepares it;

    - 'post-commit' is run after a successful commit is made.

The first two can interfere to stop the commit.  The last one is
for after-the-fact notification.

The earlier built-in commit checker is now moved to pre-commit.

Signed-off-by: Junio C Hamano <junkio@cox.net>