help: check early if we have a command, if not try a documentation topic

Before this patch, something like "git help tutorial" did not work,
people had to use "git help gittutorial" which is not very intuitive.

This patch uses the "is_git_command" function to test early if the
argument passed to "git help" is a git command, and if this is not the
case then we prefix the argument with "git" instead of "git-".

This way, things like "git help tutorial" or "git help glossary" will
work fine.

The little downside of this patch is that the "is_git_command" is a
little bit slow.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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