parse-options: typo check for unknown switches

The user specifies a long option but forgets to type the second
leading dash, we currently detect and report that fact if its first
letter is a valid short option. This is done for safety, to avoid
ambiguity between short options (and their arguments) and a long
option with a missing dash.

This diagnostic message is also helpful for long options whose first
letter is not a valid short option, however. Print it in that case,
too, as a courtesy.

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

t/t0040-parse-options.sh

@@ -236,6 +236,16 @@ test_expect_success 'detect possible typos' '
 	test_cmp typo.err output.err
 '
 
+cat > typo.err << EOF
+error: did you mean \`--ambiguous\` (with two dashes ?)
+EOF
+
+test_expect_success 'detect possible typos' '
+	test_must_fail test-parse-options -ambiguous > output 2> output.err &&
+	test ! -s output &&
+	test_cmp typo.err output.err
+'
+
 cat > expect <<EOF
 boolean: 0
 integer: 0