scripts: pre-commit: Check newly added blacklist.conf entries
    
    When blacklist.conf entries are added by merging an "upstream" branch,
    they are not checked against present commits and the repo ends up in
    inconsistent state when the patch is present and blacklisted at the same
    time.
    
    The state is checked in pre-commit hook when a (blacklisted) patch is
    added. Prevent reaching this state when adding blacklist.conf entries
    too.
    
    Using scripts/check-patch-blacklist for this check would be
    prohibitively slow (~5 minutes with 40k patches).