git/hooks: ensures the revision sent to fedora-messaging are strings not oid objects
    
    The issue was triggered when multiple commits were pushed at once at
    which point the list of revisions contained a list of OID objects
    that are not JSON serializable but can be converted to text via
    str() or by accessing their .hex attribute.
    However, when a single commit is pushed, the list of revisions
    contains just the hash of the commit pushed and thus is already
    a string. So relying on .hex will not work then.
    Using str() becomes the solution that works for both scenarios.
    
    Fixes https://pagure.io/fedora-infrastructure/issue/9247
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>