[PATCH] Simplify WC DB function start_directory_update_txn()

Hi Bert.

Any objection to me simplifying start_directory_update_txn(), as in the
attached patch? It appears that it's doing relatively a lot of work
every time just to decide whether the repos_relpath is going to be the
same as before, and thus decide whether to use a different update
statement that omits that parameter. It seems to that the work done,
and the code complexity, must far outweigh the cost of simply providing
the parameter every time. Am I missing something? I haven't profiled
it, as it looks like an obvious win.

I'll commit if no objection.

- Julian