[PATCH] doc-diff: always use oids inside worktree

[Jeff King <peff@peff.net>]

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>
---
Another fixup for jk/diff-rendered-docs, noticed when trying it out on
Eric's worktree series (which, btw, rendered as expected once I fixed
this bug ;) ).

 Documentation/doc-diff | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/Documentation/doc-diff b/Documentation/doc-diff
index 6e285e648c..c430fe7c99 100755
--- a/Documentation/doc-diff
+++ b/Documentation/doc-diff
@@ -82,7 +82,7 @@ generate_render_makefile () {
 	done
 }
 
-# render_tree <dirname> <committish>
+# render_tree <committish_oid>
 render_tree () {
 	# Skip install-man entirely if we already have an installed directory.
 	# We can't rely on make here, since "install-man" unconditionally
@@ -92,7 +92,7 @@ render_tree () {
 	# through.
 	if ! test -d "$tmp/installed/$1"
 	then
-		git -C "$tmp/worktree" checkout "$2" &&
+		git -C "$tmp/worktree" checkout --detach "$1" &&
 		make -j$parallel -C "$tmp/worktree" \
 			GIT_VERSION=omitted \
 			SOURCE_DATE_EPOCH=0 \
@@ -112,6 +112,6 @@ render_tree () {
 	fi
 }
 
-render_tree $from_oid "$from" &&
-render_tree $to_oid "$to" &&
+render_tree $from_oid &&
+render_tree $to_oid &&
 git -C $tmp/rendered diff --no-index "$@" $from_oid $to_oid
-- 
2.19.0.rc1.9.gf8a776330b