help -a: do not unnecessarily look for a repository

help -a: do not unnecessarily look for a repository

Although 'git help -a' actually doesn't need to be run inside a git
repository and uses no repository-specific information, it looks for a git
directory.  On 'git <TAB><TAB>' the bash completion runs 'git help -a' and
unnecessary 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 with the -a option.

Reported by Vincent Danjean through http://bugs.debian.org/539273

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>