git.c: show usage for accessing the git(1) help page

It is not immediately obvious how to use the `git help` system to show
the git(1) page, with its overview and its background and coordinating
material, such as environment variables.

Let's simply list it as the last few words of the last usage line.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>