Don't sync directory on Windows

Calling sync on a directory on Windows just returns "The handle is invalid"
and fails.

repo/repo.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strings"
 	"time"
 
@@ -637,6 +638,11 @@ func (h *Handler) saveBlob(w http.ResponseWriter, r *http.Request) {
 }
 
 func syncDir(dirname string) error {
+	if runtime.GOOS == "windows" {
+		// syncing a directory is not possible on windows
+		return nil
+	}
+
 	dir, err := os.Open(dirname)
 	if err != nil {
 		return err