don't os.chdir to app.py directory

Per commit msg:

this was breaking local/development runs of the form:

`    $ python webdiff/app.py a b`

I couldn't see any reason why it was present / used.
OK. It's a little strange because this whole thing currently gets run a second time on startup, and `ORIGINAL_DIR` is no longer "correct" (it gets the dir that the first run `chdir`d to).

I'll play with the cmd you have above and installing my own distro, and see if I can make something that works in all cases.

Looking at the `WERKZEUG_RUN_MAIN` env var as I do in the skip-browser-on-reload code could be the way