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.

scan.go

@@ -186,10 +186,17 @@ func syncScan(db *sql.DB, roots []string, workers int,
 // can be recognized as a possible duplicate of a tree scanned
 // separately into the same database.
 func (s *scanState) loadIndex(roots []string) error {
+	// Indexing tens of millions of records takes real time; without a
+	// display the scan looks hung before the walk begins.
+	prog := newProgress("load", -1)
+	defer prog.finish()
+
 	s.existing = make(map[string]fileMeta)
 
 	return loadFileMeta(s.db,
 		func(path string, size, mtime int64, hashed bool) {
+			prog.increment()
+
 			if underAnyRoot(path, roots) {
 				s.existing[path] = fileMeta{
 					size: size, mtime: mtime, hashed: hashed,