Show usage string for 'git show-index -h'

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

show-index.c

@@ -1,6 +1,9 @@
 #include "cache.h"
 #include "pack.h"
 
+static const char show_index_usage[] =
+"git show-index < <packed archive index>";
+
 int main(int argc, char **argv)
 {
 	int i;
@@ -8,6 +11,8 @@ int main(int argc, char **argv)
 	unsigned int version;
 	static unsigned int top_index[256];
 
+	if (argc != 1)
+		usage(show_index_usage);
 	if (fread(top_index, 2 * 4, 1, stdin) != 1)
 		die("unable to read header");
 	if (top_index[0] == htonl(PACK_IDX_SIGNATURE)) {