Improve exclusion of .git directory from copy task

I think the previous syntax for the include was not working on Windows
at least to ignore the .git directories contents. It was trying to
delete the .git dir instead of ignoring it.

This is related to #68.

src/main/java/org/ajoberstar/gradle/git/publish/GitPublishExtension.java

@@ -32,7 +32,7 @@ public GitPublishExtension(Project project) {
 
     this.contents = project.copySpec();
     this.preserve = new PatternSet();
-    this.preserve.include(".git");
+    this.preserve.include(".git/**/*");
   }
 
   public DirectoryProperty getRepoDir() {