gpg-interface: propagate exit status from gpg back to the callers

gpg-interface: propagate exit status from gpg back to the callers

When gpg-interface API unified support for signature verification
codepaths for signed tags and signed commits in mid 2015 at around
v2.6.0-rc0~114, we accidentally loosened the GPG signature
verification.

Before that change, signed commits were verified by looking for
"G"ood signature from GPG, while ignoring the exit status of "gpg
--verify" process, while signed tags were verified by simply passing
the exit status of "gpg --verify" through.  The unified code we
currently have ignores the exit status of "gpg --verify" and returns
successful verification when the signature matches an unexpired key
regardless of the trust placed on the key (i.e. in addition to "G"ood
ones, we accept "U"ntrusted ones).

Make these commands signal failure with their exit status when
underlying "gpg --verify" (or the custom command specified by
"gpg.program" configuration variable) does so.  This essentially
changes their behaviour in a backward incompatible way to reject
signatures that have been made with untrusted keys even if they
correctly verify, as that is how "gpg --verify" behaves.

Note that the code still overrides a zero exit status obtained from
"gpg" (or gpg.program) if the output does not say the signature is
good or computes correctly but made with untrusted keys, to catch
a poorly written wrapper around "gpg" the user may give us.

We could exclude "U"ntrusted support from this fallback code, but
that would be making two backward incompatible changes in a single
commit, so let's avoid that for now.  A follow-up change could do so
if desired.

Helped-by: Vojtech Myslivec <vojtech.myslivec@nic.cz>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>