progress: stop getting stuck in a nested topic during a long inner step
Convert, for example, has loops like this:
for revision in source_repo:
progress(revisions)
for file in revision:
progresss(file)
Prior to this change, we would start showing the file-level progress
when we encountered a big revision, and then we'd get stuck in showing
file-progress instead of revision progress, often producing many many
instantly-completing progress bars rather than the actually-helpful
top-level revisions bar.