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>

Documentation/technical/api-parse-options.txt

@@ -96,7 +96,8 @@ Flags are the bitwise-or of:
 	`PARSE_OPT_STOP_AT_NON_OPTION` is set, the second argument in
 	`--unknown value` will be mistakenly interpreted as a
 	non-option, not as a value belonging to the unknown option,
-	stopping the parser early.
+	the parser early.  That's why parse_options() errors out if
+	both options are set.
 
 `PARSE_OPT_NO_INTERNAL_HELP`::
 	By default, parse_options() handles `-h`, `--help` and

parse-options.c

@@ -244,6 +244,9 @@ void parse_options_start(struct parse_opt_ctx_t *ctx,
 	ctx->out  = argv;
 	ctx->cpidx = ((flags & PARSE_OPT_KEEP_ARGV0) != 0);
 	ctx->flags = flags;
+	if ((flags & PARSE_OPT_KEEP_UNKNOWN) &&
+	    (flags & PARSE_OPT_STOP_AT_NON_OPTION))
+		die("STOP_AT_NON_OPTION and KEEP_UNKNOWN don't go together");
 }
 
 static int usage_with_options_internal(const char * const *,