Get the next commit from the revision walk.

The initial call to this method is not blocking when iterating through a repo with a time-sorting mode.

Iterating with Topological or inverted modes makes the initial call blocking to preprocess the commit list, but this block should be mostly unnoticeable on most repositories (topological preprocessing times at 0.3s on the git.git repo).

The revision walker is reset when the walk is over.