Add "git branch" script

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.