sync: implement --list-cutoff to allow on disk sorting for reduced memory use

Before this change, rclone had to load an entire directory into RAM in
order to sort it so it could be synced.

With directories with millions of entries, this used too much memory.

This fixes the probem by using an on disk sort when there are more
than --list-cutoff entries in a directory.

Fixes #7974

fs/config.go

@@ -277,6 +277,11 @@ var ConfigOptionsInfo = Options{{
 	Default: false,
 	Help:    "Use recursive list if available; uses more memory but fewer transactions",
 	Groups:  "Listing",
+}, {
+	Name:    "list_cutoff",
+	Default: 1_000_000,
+	Help:    "To save memory, sort directory listings on disk above this threshold",
+	Groups:  "Sync",
 }, {
 	Name:    "tpslimit",
 	Default: 0.0,
@@ -616,6 +621,7 @@ type ConfigInfo struct {
 	Suffix                     string            `config:"suffix"`
 	SuffixKeepExtension        bool              `config:"suffix_keep_extension"`
 	UseListR                   bool              `config:"fast_list"`
+	ListCutoff                 int               `config:"list_cutoff"`
 	BufferSize                 SizeSuffix        `config:"buffer_size"`
 	BwLimit                    BwTimetable       `config:"bwlimit"`
 	BwLimitFile                BwTimetable       `config:"bwlimit_file"`