mw-to-git: check blank credential attributes via length

When writing a credential to git-credential, we omit fields
that do not have a true value. This will skip empty or
undefined fields (which we want), but will also accidentally
skip usernames or passwords which happen to have a non-true
value (e.g., "0"). Be more careful by checking for non-zero
length.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/mw-to-git/git-remote-mediawiki

@@ -207,7 +207,7 @@ sub credential_write {
 	my $credential = shift;
 	my $writer = shift;
 	while (my ($key, $value) = each(%$credential) ) {
-		if ($value) {
+		if (length $value) {
 			print $writer "$key=$value\n";
 		}
 	}