evolve: fixed lock acquire before checking state Before checking the states `repo.lock()` and `repo.wlock()` need to be acquired. This was not happening in `evolve()` earlier. This patch creates a seperate `_performevolve()` funtion which can be then called with locks acquired. This also removes the redundant lock acuquiring of `abortevolve` and `continueevolve` as lock is now taken earlier.

py3: add back a progress(0) call lost in 8664231b47ac I don't think it really matters, but it wasn't intentional, so let's just restore the old behavior.