git-send-email: add rfc2047 quoting for "=?"

For raw subjects rfc2047 quoting is needed not only for non-ASCII characters,
but also for any possible rfc2047 in it.

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Jeff King <peff@peff.net>

git-send-email.perl

@@ -908,7 +908,7 @@ sub is_rfc2047_quoted {
 sub subject_needs_rfc2047_quoting {
 	my $s = shift;
 
-	return ($s =~ /[^[:ascii:]]/);
+	return ($s =~ /[^[:ascii:]]/) || ($s =~ /=\?/);
 }
 
 sub quote_subject {