Allow '-' in config variable names

I need this in order to allow aliases of the same form as "ls-tree",
"rev-parse" etc, so that I can use

[alias]
my-cat=--paginate cat-file -p

to add a "git my-cat" command.

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