parseopt: prevent KEEP_UNKNOWN and STOP_AT_NON_OPTION from being used together

parseopt: prevent KEEP_UNKNOWN and STOP_AT_NON_OPTION from being used together

As suggested by Junio, disallow the flags PARSE_OPT_KEEP_UNKNOWN and
PARSE_OPT_STOP_AT_NON_OPTION to be turned on at the same time, as a
value of an unknown option could be mistakenly classified as a
non-option, stopping the parser early.  E.g.:

	git cmd --known --unknown value arg0 arg1

The parser should have stopped at "arg0", but it already stops at
"value".

This patch makes parse_options() die if the two flags are used in
combination.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>