One progress reporter spans every path of a snapshot. Its counts of
files and bytes processed run across all the paths, while the totals
they were divided by were reset to each path's own, so a second path
smaller than the first showed more than 100% and an ETA of `unknown`.
The totals now add up over the paths scanned so far. The rate behind
the ETA is measured from the bytes processed since the current path's
scan phase ended, so a later path's scan phase does not lower it.
SetTotalSize is renamed AddTotalSize because it now adds.
The issue says the ETA went negative; it was computed negative and
printed as `unknown`.
Model: opus-5-5
