pull: Show Estimating if we're scanning too

pull: Show Estimating if we're scanning too

The libcurl backend does all the work in the main thread/loop, which
seems to starve the idle scanning worker more.  With the libcurl
backend, we're a lot more likely to have at least one outstanding
metadata request.

But it can more easily transiently happen with libcurl that all of our current
fetches are content. To be accurate here, just show Estimating if we're scanning
too.

Closes: #654
Approved by: jlebon