checkout-index -h: show usage even in an invalid repository

checkout-index loads the index before parsing options.  Erroring out
is counterproductive at that point if the operator is hunting for a
command to recover useful data from the broken repository.

[jn: new commit message, tests]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2006-checkout-index-basic.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+test_description='basic checkout-index tests
+'
+
+. ./test-lib.sh
+
+test_expect_success 'checkout-index --gobbledegook' '
+	test_expect_code 129 git checkout-index --gobbledegook 2>err &&
+	grep "[Uu]sage" err
+'
+
+test_expect_success 'checkout-index -h in broken repository' '
+	mkdir broken &&
+	(
+		cd broken &&
+		git init &&
+		>.git/index &&
+		test_expect_code 129 git checkout-index -h >usage 2>&1
+	) &&
+	grep "[Uu]sage" broken/usage
+'
+
+test_done