Change default checkout strategy from FORCE to SAFE

Since we are able to give our own git checkout options,
the default git checkout strategy will be the same as
initialized in a new git_checkout_options struct.

src/worktree.c

@@ -407,8 +407,6 @@ int git_worktree_add(git_worktree **out, git_repository *repo,
 	/* Checkout worktree's HEAD */
 	if (opts != NULL)
 		memcpy(&coopts, &wtopts.checkout_opts, sizeof(coopts));
-	else
-		coopts.checkout_strategy = GIT_CHECKOUT_FORCE;
 	if ((err = git_checkout_head(wt, &coopts)) < 0)
 		goto out;