branch -h: show usage even in an invalid repository

branch -h: show usage even in an invalid repository

There is no need for "git branch -h" to try to access a repository.

In the spirit of v1.6.6-rc0~34^2~3 (Let 'git <command> -h' show usage
without a git dir, 2009-11-09).  This brings git one step closer to
passing the following (automatically verifiable) test:

 Before any repository access (aside from git_config()), a
 function from the setup_git_directory_* family has been run

and thus one step closer to being able to use an automatic repository
access checker.

[jn: simplified; new commit message, test]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>