update-index -h: show usage even with corrupt index

When trying to fix up a corrupt repository, one might prefer that
"update-index -h" print an accurate usage message and exit rather
than reading the repository and complaining about the corruption.

[jn: with rewritten log message and 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>

builtin/update-index.c

@@ -589,6 +589,9 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	int lock_error = 0;
 	struct lock_file *lock_file;
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage(update_index_usage);
+
 	git_config(git_default_config, NULL);
 
 	/* We can't free this memory, it becomes part of a linked list parsed atexit() */

t/t2107-update-index-basic.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+test_description='basic update-index tests
+
+Tests for command-line parsing and basic operation.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'update-index --nonsense fails' '
+	test_must_fail git update-index --nonsense 2>msg &&
+	cat msg &&
+	test -s msg
+'
+
+test_expect_failure 'update-index --nonsense dumps usage' '
+	test_expect_code 129 git update-index --nonsense 2>err &&
+	grep "[Uu]sage: git update-index" err
+'
+
+test_expect_success 'update-index -h with corrupt index' '
+	mkdir broken &&
+	(
+		cd broken &&
+		git init &&
+		>.git/index &&
+		test_expect_code 129 git update-index -h >usage 2>&1
+	) &&
+	grep "[Uu]sage: git update-index" broken/usage
+'
+
+test_done