lib/versioner: Fix external versioner command specification on Windows (fixes #4560)

lib/versioner/external.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"os"
 	"os/exec"
+	"runtime"
 	"strings"
 
 	"github.com/syncthing/syncthing/lib/fs"
@@ -30,6 +31,10 @@ type External struct {
 func NewExternal(folderID string, filesystem fs.Filesystem, params map[string]string) Versioner {
 	command := params["command"]
 
+	if runtime.GOOS == "windows" {
+		command = strings.Replace(command, `\`, `\\`, -1)
+	}
+
 	s := External{
 		command:    command,
 		filesystem: filesystem,