doc-diff: let `render_tree()` take an explicit directory name

doc-diff: let `render_tree()` take an explicit directory name

In `render_tree()`, `$1` is documented to be the commit-ish/oid and we
use it as that with `git checkout`, but we mostly use it to form the
name of various directories. To separate these concerns, and because we
are about to construct the directory names a bit differently, take two
distinct arguments instead.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>