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