tree: look for conflicts in the new tree when updating

We look at whether we're trying to replace a blob with a tree during the
update phase, but we fail to look at whether we've just inserted a blob
where we're now trying to insert a tree.

Update the check to look at both places. The test for this was
previously succeeding due to the bu where we did not look at the sorted
output.

src/tree.c

@@ -1200,6 +1200,9 @@ int git_tree_create_updated(git_oid *out, git_repository *repo, git_tree *baseli
 
 			last = git_array_last(stack);
 			entry = last->tree ? git_tree_entry_byname(last->tree, component.ptr) : NULL;
+			if (!entry)
+				entry = treebuilder_get(last->bld, component.ptr);
+
 			if (entry && git_tree_entry_type(entry) != GIT_OBJ_TREE) {
 				giterr_set(GITERR_TREE, "D/F conflict when updating tree");
 				error = -1;