Change the method to retrieve the first commit from git branch
    
    This method is used by scm_request_processor and the previous `git rev-list`
    implementation failed in one case, where there was one than one commit without
    parents in the same branch.
    This will rather use the `git log` and picks the hash of the oldest commit.
    
    This commit also further enhances the error message posted by bot.
    
    Signed-off-by: Michal Konečný <mkonecny@redhat.com>