Add alias `new-branch` and `nb` to git

The alias `new-branch` is equivalent to `checkout -b`. This creates a new branch
with the name provided as the first argument `$1`, and switches to the new
branch. `nb` is an alias of `new-branch`.
Add alias `undo` to git

The alias `undo` is equivalent to `reset --soft HEAD~1`. This resets to the last
commit, while keeping changes to files and keeping them in the index. This is
useful when something is missing or wrong with the original commit.