Accept git aliases outside a git repository

Accept git aliases outside a git repository

af05d67 (Always set *nongit_ok in setup_git_directory_gently(),
2008-03-25) had a change from the patch originally submitted that resulted
in disabling aliases outside a git repository.

It turns out that some people used "alias.fubar = diff --color-words" in
$HOME/.gitconfig to use non-index diff (or any command that do not need
git repository) outside git repositories, and this change broke them,
so this resurrects the support for such usage.

Signed-off-by: Junio C Hamano <gitster@pobox.com>