Add examples for git-log documentation and others.

Add examples for git-log documentation and others.

I don't think people really follow the links or think very abstractly at
all in the first place.

So I was thinking more of some explicit examples. I actually think every
command should have an example in the man-page, and hey, here's a patch to
start things off.

Of course, I'm not exactly "Mr Documentation", and I don't know that this
is the prettiest way to do this, but I checked that the resulting html and
man-page seems at least reasonable.

And hey, if the examples look like each other, that's just because I'm
also not "Mr Imagination".

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>