gpg-interface: check gpg signature creation status

gpg-interface: check gpg signature creation status

When we create a signature, it may happen that gpg returns with
"success" but not with an actual detached signature on stdout.

Check for the correct signature creation status to catch these cases
better. Really, --status-fd parsing is the only way to check gpg status
reliably. We do the same for verify already.

Signed-off-by: Michael J Gruber <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>