Prevent bogus paths from being added to the index.

With this one, it's now a fatal error to try to add a pathname
that cannot be added with "git add", i.e.

	[torvalds@g5 git]$ git add .git/config
	fatal: unable to add .git/config to index

and

	[torvalds@g5 git]$ git add foo/../bar
	fatal: unable to add foo/../bar to index

instead of the old "Ignoring path xyz" warning that would end up
silently succeeding on any other paths.

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