Add git-commit-message to construct commit message incrementally as you work

This adds the `git commit-message` command with actions: add, edit, clear, and show (the default).  You can use it to build up a commit message while you are working, simply running:

    git commit-message add A quick reminder

When you eventually do the `git commit`, the message that you have accumulated will be the seed for the commit message to be used.

Note that this sets up prepare-commit-msg and post-commit hooks for you when used so that it can inject the commit message and clear it out after the commit.