I commited fast_bottom-up.patch to fix the regression of os.walk().

> Could you please add a test based on my example (i.e. converting symlinks to a directory during walking) and may be other (creating new directory and adding it to the dirs list)?

Sorry, I'm not interested to write new tests.

> Is it worth to mention in the os.walk() doc that topdown=False can be faster?

I didn't touch the doc yet.

I keep the issue open a few weeks to see if someone is interested to optimize os.walk() further, write new tests, enhance the doc, etc.