git help foo invokes man git-foo if foo is a git command, otherwise it
invokes man gitfoo. 'help' is not a git command, but the manual page is
called git-help, so add this special exception.
Signed-off-by: Miklos Vajna <vmiklos@frugalware.org> Acked-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>