Ensure GC.deleteOrphans() can delete read-only orphaned files on Windows

Reported in https://www.eclipse.org/lists/jgit-dev/msg04005.html

Change-Id: I140a49a7f65a76aa2b67ec8d286a3d2506ae499a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java

@@ -978,7 +978,8 @@ public class GC {
 			} else {
 				if (base == null || !n.startsWith(base)) {
 					try {
-						Files.delete(packDir.resolve(n));
+						FileUtils.delete(packDir.resolve(n).toFile(),
+								FileUtils.RETRY | FileUtils.SKIP_MISSING);
 					} catch (IOException e) {
 						LOG.error(e.getMessage(), e);
 					}