Wrong exit code on failed SSH signing

Hello!

There seems to be a bug in the sign_buffer_ssh function in
gpg-interface.c: a possible exit code of ssh-keygen is 255, which is
returned as-is by sign_buffer_ssh. The problem is that, for example,
the function build_tag_object in builtin/tag.c considers only negative
values as a failure. Since 255 >= 0, the error message "unable to sign
the tag" is not emitted and git exits normally with zero exit code. It
might be enough to return -1 in sign_buffer_ssh if ret is not zero.

I am sorry if this has already been reported or taken care of. Thank you.

Best regards,
Sergey