s3: reduce the default --s3-copy-cutoff to < 5GB

The maximum value for the --s3--copy-cutoff should be 5GiB as tested
with AWS S3.

However b2 have implemented this as 5GB rather than 5GiB so having the
default at 5 GiB makes the b2s3 server side copy of a large file by
default.

This patch sets the default to 4768 MiB which is slightly less than
5GB.

This should have very little effect on anything.

If in future rclone can lower this limit more if Copy can multithread.

See: https://forum.rclone.org/t/copying-files-within-a-b2-bucket/16680/76

backend/s3/s3.go

@@ -1064,10 +1064,12 @@ This option controls how often unused buffers will be removed from the pool.`,
 
 // Constants
 const (
-	metaMtime           = "Mtime"                // the meta key to store mtime in - eg X-Amz-Meta-Mtime
-	metaMD5Hash         = "Md5chksum"            // the meta key to store md5hash in
-	maxSizeForCopy      = 5 * 1024 * 1024 * 1024 // The maximum size of object we can COPY
-	maxUploadParts      = 10000                  // maximum allowed number of parts in a multi-part upload
+	metaMtime   = "Mtime"     // the meta key to store mtime in - eg X-Amz-Meta-Mtime
+	metaMD5Hash = "Md5chksum" // the meta key to store md5hash in
+	// The maximum size of object we can COPY - this should be 5GiB but is < 5GB for b2 compatibility
+	// See https://forum.rclone.org/t/copying-files-within-a-b2-bucket/16680/76
+	maxSizeForCopy      = 4768 * 1024 * 1024
+	maxUploadParts      = 10000 // maximum allowed number of parts in a multi-part upload
 	minChunkSize        = fs.SizeSuffix(1024 * 1024 * 5)
 	defaultUploadCutoff = fs.SizeSuffix(200 * 1024 * 1024)
 	maxUploadCutoff     = fs.SizeSuffix(5 * 1024 * 1024 * 1024)