Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/25616

**Problem**

It is possible to attach multiple PGP signatures to the commit. But
git does not support that (see:
https://github.com/git/git/commit/da6cf1b3360eefdce3dbde7632eca57177327f37).

**Solution**

Reject multiple PGP signatures for commits (even if they valid ones)
to match `git verify-commit` behavior.

Show an error message to describe why we mark commit as unverified.