lib/scanner: More sensible debug output (#8596)

Previous debug input didn't really give enough info to show what was
happening, while it also printed full block lists which are enormously
verbose. Now it consistently prints 1. what it sees on disk, 2. what it
got from CurrentFile (without blocks), 3. the action taken on that file.

lib/scanner/blockqueue.go

@@ -98,6 +98,8 @@ func (ph *parallelHasher) hashFiles(ctx context.Context) {
 				return
 			}
 
+			l.Debugln("started hashing:", f)
+
 			if f.IsDirectory() || f.IsDeleted() {
 				panic("Bug. Asked to hash a directory or a deleted file.")
 			}
@@ -120,6 +122,7 @@ func (ph *parallelHasher) hashFiles(ctx context.Context) {
 				f.Size += int64(b.Size)
 			}
 
+			l.Debugln("completed hashing:", f)
 			select {
 			case ph.outbox <- ScanResult{File: f}:
 			case <-ctx.Done():