config parsing options: allow one flag multiple times

config parsing options: allow one flag multiple times

This task emerged from b04ba2bb (parse-options: deprecate OPT_BOOLEAN,
2011-09-27).

This commit introduces a change for the users, after this patch
you can pass one of the config level flags multiple times:
Before:
	$ git config --global --global --list
	error: only one config file at a time.
	usage: ...

Afterwards this will work. This is due to the following check in the code:
	if (use_global_config + use_system_config + use_local_config +
	    !!given_config_file + !!given_config_blob > 1) {
		error("only one config file at a time.");
		usage_with_options(builtin_config_usage, builtin_config_options);
	}

With OPT_BOOL instead of OPT_BOOLEAN the variables use_global_config,
use_system_config, use_local_config will only have the value 0 if the
command line option was not passed or 1 no matter how often the
respective command line option was passed.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>