[PATCH v2] git-commit --patch

This patch-set adds support for git-commit --patch, and tidies up some of the rough edges of git commit --interactive.

The motivation is to support my current workflow, which goes something like:

1. Hack out the basic structure of the feature that I'm working on, until I have
   something that looks like it will work.
2. Split this into several commits with a more logical flow (i.e. some that add
   support for the techniques I want to use for the actual feature, then the
   feature itself).
3. Start fleshing out the implementation, and bug-fixing, with lots of
   git-commit --fixup so that the changes end up in the right commit.
4. At the end of the day, a big rebase -i to make the history readable.

This is just about doable with git-add -p, or git-commit --interactive, but it's very inefficient. I take the presence of git commit --fixup to imply that other people are doing similar, if less extreme things, so assume that they would like a git-commit -p too.

Conrad