Show progress while loading the record index

On a database with tens of millions of records, indexing the existing
rows before the walk takes real single-core time with no output,
which is indistinguishable from a hang. Give the load its own
spinner, and render every phase display the moment the phase starts
instead of waiting for its first completed item.

progress.go

@@ -65,6 +65,9 @@ func newProgress(label string, total int64) *progress {
 		progressbar.OptionSetItsString("files"),
 		progressbar.OptionSetElapsedTime(true),
 		progressbar.OptionThrottle(barThrottle),
+		// Render at zero immediately: a phase must be visible the
+		// moment it starts, even before its first item completes.
+		progressbar.OptionSetRenderBlankState(true),
 	}
 	if total >= 0 {
 		opts = append(opts,