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>

t/t0040-parse-options.sh

@@ -33,6 +33,8 @@ Magic arguments
     --quux                means --quux
     -NUM                  set integer to NUM
     +                     same as -b
+    --ambiguous           positive ambiguity
+    --no-ambiguous        negative ambiguity
 
 Standard options
     --abbrev[=<n>]        use <n> digits to display SHA-1s
@@ -315,4 +317,22 @@ test_expect_success 'OPT_NUMBER_CALLBACK() works' '
 	test_cmp expect output
 '
 
+cat >expect <<EOF
+boolean: 0
+integer: 0
+timestamp: 0
+string: (not set)
+abbrev: 7
+verbose: 0
+quiet: no
+dry run: no
+file: (not set)
+EOF
+
+test_expect_success 'negation of OPT_NONEG flags is not ambiguous' '
+	test-parse-options --no-ambig >output 2>output.err &&
+	test ! -s output.err &&
+	test_cmp expect output
+'
+
 test_done