parse-opt: ignore negation of OPT_NONEG for ambiguity checks

parse_long_opt always matches both --opt and --no-opt for any option
"opt", and only get_value checks whether --no-opt is actually valid.
Since the options for git branch contains both "no-merged" and "merged"
there are two matches for --no-merge, but no exact match.  With this
patch the negation of a NONEG option is rejected earlier, but it changes
the error message from "option `no-opt' isn't available" to "unknown
option `no-opt'".

[jk: added test]

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

parse-options.c

@@ -230,6 +230,9 @@ is_abbreviated:
 				abbrev_flags = flags;
 				continue;
 			}
+			/* negation allowed? */
+			if (options->flags & PARSE_OPT_NONEG)
+				continue;
 			/* negated and abbreviated very much? */
 			if (!prefixcmp("no-", arg)) {
 				flags |= OPT_UNSET;