Show usage string for 'git fast-import -h'

Let "git fast-import -h" (with no other arguments) print usage
before exiting, even when run outside any repository.

Cc: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

fast-import.c

@@ -2405,6 +2405,9 @@ int main(int argc, const char **argv)
 
 	git_extract_argv0_path(argv[0]);
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage(fast_import_usage);
+
 	setup_git_directory();
 	git_config(git_pack_config, NULL);
 	if (!pack_compression_seen && core_compression_seen)