Add "git branch" script

You can use it as

git branch <branchname> [start-point]

and it creates a new branch of name <branchname>.  If a starting point
is specified, that will be where the branch is created, otherwise it
will be created at the current HEAD.

The sequence

git branch xyz abc
git checkout xyz

can also be written as

git checkout -b xyz abc

as per the previous commit.