git-add: Add support for --, documentation, and test.

git-add: Add support for --, documentation, and test.

This adds support to git-add to allow the common -- to separate
command-line options and file names. It adds documentation and a new
git-add test case as well.

[jc: this should apply to 1.2.X maintenance series, so I reworked
 git-ls-files --error-unmatch test. ]

Signed-off-by: Junio C Hamano <junkio@cox.net>