ls-files -h: show usage even with corrupt index

Part of a campaign to avoid git <command> -h being distracted by
access to the repository.  A caller hoping to use "git ls-files"
with an alternate index as part of a repair operation may well use
"git ls-files -h" to show usage while planning it out.

[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/ls-files.c

@@ -530,6 +530,9 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 		OPT_END()
 	};
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(ls_files_usage, builtin_ls_files_options);
+
 	memset(&dir, 0, sizeof(dir));
 	prefix = cmd_prefix;
 	if (prefix)

t/t3004-ls-files-basic.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+test_description='basic ls-files tests
+
+This test runs git ls-files with various unusual or malformed
+command-line arguments.
+'
+
+. ./test-lib.sh
+
+>empty
+
+test_expect_success 'ls-files in empty repository' '
+	git ls-files >actual &&
+	test_cmp empty actual
+'
+
+test_expect_success 'ls-files with nonexistent path' '
+	git ls-files doesnotexist >actual &&
+	test_cmp empty actual
+'
+
+test_expect_success 'ls-files with nonsense option' '
+	test_expect_code 129 git ls-files --nonsense 2>actual &&
+	grep "[Uu]sage: git ls-files" actual
+'
+
+test_expect_success 'ls-files -h in corrupt repository' '
+	mkdir broken &&
+	(
+		cd broken &&
+		git init &&
+		>.git/index &&
+		test_expect_code 129 git ls-files -h >usage 2>&1
+	) &&
+	grep "[Uu]sage: git ls-files " broken/usage
+'
+
+test_done