treecache: write the raw id not the object

We explicitly want to write on the id data, not the beginning of the
object data, which may contain other information in the future.

src/libgit2/tree-cache.c

@@ -263,7 +263,7 @@ static void write_tree(git_str *out, git_tree_cache *tree)
 	git_str_printf(out, "%s%c%"PRIdZ" %"PRIuZ"\n", tree->name, 0, tree->entry_count, tree->children_count);
 
 	if (tree->entry_count != -1)
-		git_str_put(out, (const char *) &tree->oid, GIT_OID_RAWSZ);
+		git_str_put(out, (char *)&tree->oid.id, GIT_OID_RAWSZ);
 
 	for (i = 0; i < tree->children_count; i++)
 		write_tree(out, tree->children[i]);