t5334: document that git push --signed=1 does not work

When accepting booleans as command-line or config options throughout
Git, there are several documented synonyms for true and false.
However, one particular user is slightly broken: `git push --signed=..`
does not understand the integer synonyms for true and false.

This is hardly wanted. The --signed option has a different notion of
boolean than all other arguments and config options, including the
config option corresponding to it, push.gpgSign.

Add a test documenting the failure to handle --signed=1.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Martin Ågren

t/t5534-push-signed.sh

@@ -71,6 +71,13 @@ test_expect_success 'push --signed fails with a receiver without push certificat
 	test_i18ngrep "the receiving end does not support" err
 '
 
+test_expect_failure 'push --signed=1 is accepted' '
+	prepare_dst &&
+	mkdir -p dst/.git/hooks &&
+	test_must_fail git push --signed=1 dst noop ff +noff 2>err &&
+	test_i18ngrep "the receiving end does not support" err
+'
+
 test_expect_success GPG 'no certificate for a signed push with no update' '
 	prepare_dst &&
 	mkdir -p dst/.git/hooks &&