[PATCH 1/1] verify-tag/verify-commit should exit unsuccessfully when signature is not trusted

Hello,

me and my colleague are struggling with automation of verifying git
repositories and we have encountered that git verify-commit and
verify-tag accepts untrusted signatures and exit successfully.

We have done some investigation of the GPG verification changes in git
repository which I includes in this patch message. GPG results
`TRUST_NEVER` and `TRUST_UNDEFINED` in raw output is treated as
untrusted in git (U) and should not be accepted in verify-commit and
verify-tag command.


In 434060ec6d verify-tag and verify-commit was centralized into
check_signature function and good (G) and untrusted (U) signatures were
marked as valid and exited successfully. In this commit it is
incorrectly stated that this behavior is adopted from older verify-tag
function however original verify-tag behavior was to return exit code
from gpg process itself (removed in a4cc18f29).

Also rejecting untrusted (U) signature is the pull/merge with
--verify-signatures behavior (defined in builtin/merge.c cmd_merge
function and presented in eb307ae7bb).

The behavior of merge/pull --verify-signatures and
verify-commit/verify-tag should be the same.


With regards,
Vojtech Myslivec and Karel Koci