Add some tabbing keymaps to Vim.

Allows easier use of tabs.
Add `git prepare` and `git blog`

To conform with git_ship, `git blog` is the new alias for logging.
It's a combo of the former `git l` and `git lol` log aliases. To
maintain backwards compatibility, `git l` has been temporarily
aliased to `git blog`.

`git prepare` simply rebases interactively to 'origin/master'.
This will clear up any commits on master that you need to change
or rewrite before pushing to origin.