help: Do not unnecessarily look for a repository

help: Do not unnecessarily look for a repository

Although 'git help' actually doesn't need to be run inside a git
repository and uses no repository-specific information, it looks for a git
directory.  Searching for a git directory can be annoying in auto-mount
environments.  With this commit, 'git help' no longer searches for a
repository when run without any options.

7c3baa9 originally modified 'git help -a' to not require a repository.
This applies the same fix for 'git help'.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>