[PATCH] Do not create bogus branch from flag to git branch

If you run `git branch --help', you will unexpectedly have created a new
branch named "--help".  This simple patch adds logic and a usage
statement to catch this and similar problems, and adds a testcase for it.

Signed-off-by: Amos Waterland <apw@rossby.metr.ou.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>