doc-diff: always use oids inside worktree

The doc-diff script immediately resolves its two endpoints to actual object ids, so that we can reuse cached results even if they appear under a different name. But we still use the original name the user fed us when running "git checkout" in our temporary worktree. This can lead to confusing results: - the namespace inside the worktree is different than the one outside. In particular, "./doc-diff origin HEAD" will resolve HEAD inside the worktree, whose detached HEAD will be pointing at origin! As a result, such a diff would always be empty. - worse, we will store this result under the oid we got by resolving HEAD in the main worktree, thus polluting our cache - we didn't pass --detach, which meant that using a branch name would cause us to actually check out that branch, making it unavailable to other worktrees. We can solve this by feeding the already-resolved object id to git-checkout. That naturally forces a detached HEAD, but just to make clear our expectation, let's explicitly pass --detach. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>