lsjson: add --no-mimetype flag, speed up lsf

Before this changed we unconditionally fetched the MimeType. On Some
backends like s3 and swift this takes an extra transaction which meant
that `lsf` on those backends was needlessly slow.

This adds an internal option so `lsf` can declare whether it wants
MimeTypes or not depending on whether the user asked for them and an
external flag `--no-mimetype` for `lsjson`.

See: https://forum.rclone.org/t/reliably-setup-incremental-updates/14006/8

cmd/lsf/lsf.go

@@ -166,10 +166,11 @@ func Lsf(ctx context.Context, fsrc fs.Fs, out io.Writer) error {
 	list.SetDirSlash(dirSlash)
 	list.SetAbsolute(absolute)
 	var opt = operations.ListJSONOpt{
-		NoModTime: true,
-		DirsOnly:  dirsOnly,
-		FilesOnly: filesOnly,
-		Recurse:   recurse,
+		NoModTime:  true,
+		NoMimeType: true,
+		DirsOnly:   dirsOnly,
+		FilesOnly:  filesOnly,
+		Recurse:    recurse,
 	}
 
 	for _, char := range format {
@@ -188,6 +189,7 @@ func Lsf(ctx context.Context, fsrc fs.Fs, out io.Writer) error {
 			list.AddID()
 		case 'm':
 			list.AddMimeType()
+			opt.NoMimeType = false
 		case 'e':
 			list.AddEncrypted()
 			opt.ShowEncrypted = true