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>

git-add.sh

@@ -14,6 +14,10 @@ while : ; do
     -v)
 	verbose=--verbose
 	;;
+    --)
+	shift
+	break
+	;;
     -*)
 	usage
 	;;