tree.h API: expose read_tree_1() as read_tree_at()

tree.h API: expose read_tree_1() as read_tree_at()

Rename the static read_tree_1() function to read_tree_at(). This
function works just like read_tree_recursive(), except you provide
your own strbuf.

This step doesn't make much sense now, but in follow-up commits I'll
remove the base/baselen/stage arguments to read_tree_recursive(). At
that point an anticipated in-tree user[1] for the old
read_tree_recursive() couldn't provide a path to start the
traversal.

Let's give them a function to do so with an API that makes more sense
for them, by taking a strbuf we should be able to avoid more casting
and/or reallocations in the future.

1. https://lore.kernel.org/git/xmqqft106sok.fsf@gitster.g

Signed-off-by: Junio C Hamano <gitster@pobox.com>