Check if post was modified by editor different than post author

Question

IMO, the only draw-back of the default Post Revisions is that it stores the auto-save actions too. But, it does have a history of what changed and who changed, and seems to fullfil the OP requirements.

If some change needs to be applied to this default Meta Box, it should be removed and recreated, as it seems no hook is available. Maybe a simple jQuery is easier to remove the Autosave entries:

add_action( 'admin_head-post-new.php', 'remove_autosave_revisions_wpse_75651' );
add_action( 'admin_head-post.php', 'remove_autosave_revisions_wpse_75651' );

function remove_autosave_revisions_wpse_75651()
{
    ?>
    <script language="javascript" type="text/javascript">
    jQuery(document).ready(function($) 
    {
        $("ul.post-revisions").find("li:contains('Autosave')").remove();
    });
    </script>
    <?php
}

Case a Custom Meta Box is intended, check the following sample:

add_action( 'add_meta_boxes', 'add_custom_box_wpse_75651' );
add_action( 'save_post', 'save_postdata_wpse_75651', 10, 2 );

function add_custom_box_wpse_75651() 
{
    add_meta_box( 
        'sectionid_wpse_75651',
        __( 'Post History' ),
        'inner_custom_box_wpse_75651',
        'post' 
    );
}

function inner_custom_box_wpse_75651( $post ) 
{
    $history = get_post_meta( $post->ID, '_history', true );
    if( $history )
    {
        $reverse_history = array_reverse( $history );
        echo "
            <table class="widefat">
                <thead>
                    <tr>
                        <th style="width:20%">Modified by</th>
                        <th>Date</th>
                    </tr>
                </thead>
                <tfoot>
                    <tr>
                    <th style="width:20%">Modified by</th>
                    <th>Date</th>
                    </tr>
                </tfoot>
                <tbody>";

        foreach( $reverse_history as $h )
        {
            echo "
               <tr>
                  <td>
                    {$h[0]}
                  </td>
                  <td>
                    {$h[1]}
                 </td>
               </tr>            
            ";
        }

        echo "</tbody>
                    </table>
                ";
    }
}   

function save_postdata_wpse_75651( $post_id, $post_object ) 
{
    // Block auto-save
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )  
        return;

    // Block revisions
    if ( 'revision' == $post_object->post_type )
        return;

    // Check post type
    if ( 'post' == $post_object->post_type ) 
    {
        global $current_user;
        $history = get_post_meta( $post_id, '_history', true );
        $history[] = array( $current_user->user_login, current_time( 'mysql' ) );
        update_post_meta( $post_id, '_history', $history );
    }   
}

Which results in:

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