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>

t/t3700-add.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+#
+# Copyright (c) 2006 Carl D. Worth
+#
+
+test_description='Test of git-add, including the -- option.'
+
+. ./test-lib.sh
+
+test_expect_success \
+    'Test of git-add' \
+    'touch foo && git-add foo'
+
+test_expect_success \
+    'Post-check that foo is in the index' \
+    'git-ls-files foo | grep foo'
+
+test_expect_success \
+    'Test that "git-add -- -q" works' \
+    'touch -- -q && git-add -- -q'
+
+test_done