sync: allow --max-backlog to be -ve meaning as large as possible

docs/content/docs.md

@@ -745,6 +745,9 @@ time and make `--order-by` work more accurately.
 Setting this small will make rclone more synchronous to the listings
 of the remote which may be desirable.
 
+Setting this to a negative number will make the backlog as large as
+possible.
+
 ### --max-delete=N ###
 
 This tells rclone not to delete more than N files.  If that limit is

fs/sync/pipe.go

@@ -2,6 +2,7 @@ package sync
 
 import (
 	"context"
+	"math/bits"
 	"strconv"
 	"strings"
 	"sync"
@@ -30,6 +31,9 @@ type pipe struct {
 }
 
 func newPipe(orderBy string, stats func(items int, totalSize int64), maxBacklog int) (*pipe, error) {
+	if maxBacklog < 0 {
+		maxBacklog = (1 << (bits.UintSize - 1)) - 1 // largest posititive int
+	}
 	less, fraction, err := newLess(orderBy)
 	if err != nil {
 		return nil, fserrors.FatalError(err)