Make "git help" react to window size correctly

Make "git help" react to window size correctly

Currently the git "show commands" function will react to the environment
variable COLUMNS, or just default to a width of 80 characters.

That's just soo eighties. Nobody sane sets COLUMNS any more, unless they
need to support some stone-age software from before the age of steam
engines, SIGWINCH and TIOCGWINSZ.

So get with the new century, and use TIOCGWINSZ to get the terminal size.

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