walkchangerevs gone -- breaks Trac plugin

[Alexis S. L. Carvalho]

Thus spake Georg:
> with latest tip the Trac plugin does not work any more.  It wants to import
> walkchangerevs which seems to be gone.
> 
> Are there suggestions on how to replace the walkchangerevs API?

It moved from commands.py to cmdutil.py (and now requires an extra
argument - see the log function in commands.py).

Alexis