help: introduce option --exclude-guides

help: introduce option --exclude-guides

Introduce option --exclude-guides to the help command.  With this option
being passed, "git help" will open man pages only for actual commands.

Since we know it is a command, we can use function help_unknown_command
to give the user advice on typos.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>